I can see a few questions and potential situations cropping up here. I'm going to try my hand at answering them all, and I apologize in advance if I get a little long-winded, but this may hopefully save some headaches out there, and/or clear up some confusion.
FireWire is somewhat more "robust" in terms of delivering bus power than USB. Mind you, that doesn't mean it's perfect, nor is it unlimited; it's just a bit better. Having said that...
FireWire comes in two basic configurations: six pin, and four pin. Now, there's only data going across four of the pins, so in that they're the same. However, it's the extra two pins of the six pin ports/cables which carry the power.
Apple has always used the six pin IEEE 1394 port on their desktops and laptops for FireWire 400. This is the fancy way of saying they give you the nice, fully-powered version of the port. That's a good thing.
Different FireWire devices have different kinds of ports. In general, for our purposes here, storage devices pretty much all come with the six pin port, since it's really not all that big, and usually those devices are large enough and overall relatively uncomplicated enough that nothing would be gained in going with a smaller port connector. However, this doesn't necessarily mean the device itself can be powered via the bus (aka "bus powered"). It basically depends on it's power draw.
From my experience, I can tell you that full-size (that is, standard 3.5") hard drives require a dedicated power supply, and notebook-size (that is, 2") hard drives require less and can be successfully bus-powered. So if you're looking for a strictly bus-powered HDD, then you'll want to go with an external 2" hard drive.
Now, you do need to be a bit careful here when looking around on the Internet, as sometimes a given manufacturer uses an identically-designed case for both 3.5" and 2" hard drive enclosures. Yes, the cases are of two different physical sizes, but they otherwise look identical (and promo photos often present nothing for a sense of scale). So be very careful that what you're seeing is what you think you're looking at. DO NOT ASSUME!
Also, it bears mentioning that notebook hard drives do not come in all the same flavors as their desktop counterparts. For instance, their capacities won't be as big, you probably won't find a SATA HDD mechanism, etc., etc. However, they do have both 5400 and 7200 RPM drives available.
If you're going to be using this in a highly mobile environment, please remember that HDDs, all the latest protection and inventions notwithstanding, are still hard drives. They still spin, and they can still get damaged by sudden or sharp movements. Tread with caution in the mobile use of them.
Lastly, you also should know that, even if FireWire is a superior solution for certain technical reasons, the connector design itself was really not intended for such mobile environment use. What I mean by that is simply this: the darn thing can come unplugged pretty easily. I know: I've corrupted data files this way. Inferior as the interface is, the USB connector just happens to be a bit more mechanically stable and not as readily prone to being disconnected accidentally. Them's the brakes, as they say.
Now, with regard to portable hard drives and Windows systems...
Well, since this is a Mac forum, I have to imagine there's at least a chance this drive will get moved between Windows and Mac OS computers. I'd like to say a few things on this, too.
First, yes absolutely, Windows XP will support HDDs connected via either FireWire or USB. That itself is a non-issue.
What is of far more concern is the filesystem used on the drive.
If the HDD is 100% certain never to be connected to anything but a Windows NT or XP or Server 200x computer, then by all means format the thing with NTFS.
On the other hand, if you need to trade the drive between Windows and anything else, then your absolute best bet is to use FAT32.
What's the reason for this? I know the more technically astute or well-traveled people here will want to jump up and shout "Oh, I know the answer to this one! I know the answer!" (Is anyone else here having Welcome Back, Kotter flashbacks besides me? Well, anyhow...)
Here's the deal. Yes, both FAT32 and NTFS are trademarked and copyrighted by Microsoft. However, FAT32 is simply an improved version of FAT16, and FAT16 has been around practically since Washington sailed the Delaware. In other words, almost every OS out there fully supports FAT16 and FAT32.
On the other hand, NTFS was kept far more secret and far more restricted (legally, and in other ways) by Microsoft. It was their corporate filesystem. It was intended to be secure both for data integrity and intrusion prevention purposes. Now, one can make any kind of argument they want about those two factors; nevertheless NTFS is sufficiently locked down by Microsoft that support for it by anything other than an "NT-class" Microsoft operating system is limited at best.
For any filesystem, there is a driver to support it. FAT, NTFS, HFS+, EXT3, ReiserFS, what-have-you. However, the driver(s) available for NTFS only fully support reading data from the drive, with some other limitations (mostly having to do with encrypted or otherwise-protected data, etc.) The support out there for actually writing to an NTFS drive is, well, shaky at best.
Conversely, in typical Microsoft-first, Microsoft-last, and fsck-everyone-else fashion, Windows doesn't natively support anything other than FATxx and NTFS filesystems. This means that if you decided to use, say, HFS+ or one of it's more recent variants, you'll need to buy special adaptive software for Windows. It's a hassle, costs money, and is completely unnecessary except for very specialized, usually "corporate" reasons.
FAT32, which I recommended above, is fully supported and will allow the drive to be read from and written to by Windows 9x, WinNT 4+, WinXP, Win200x Server; in addition to pretty much all GNU/Linux distributions, and of course, Mac OS 9.xx and Mac OS X. And, just as would be the case with NTFS, you simply have to be mindful of limitations in naming files. Basically, if you stick with 0-9, A-Z, spaces and the - (hyphen), and make sure you have the ".xxx" extension on the end of the file name, you should have absolutely no interoperability issues.
One last and (I know you're hoping I'll quit soon!) final note:
Mac OS X (and Windows, but to a much lesser degree) makes use of special, normally invisible files, called "metafiles". They contain various extra data on the files themselves (icon previews, window view and organization preferences, etc.) When you access a folder or create one, when you write files or modify them, etc., these files get created. Normally, on the Mac, since you can't see them, it's no big deal. However, Microsoft doesn't respect their hidden nature, and shows them for all the world to see.
I mostly mention this as an afterthought because you may wonder what they are, or get confused which one is the real deal, depending on how many files you're dealing with or for other reasons. They are not required and can be either ignored or manually deleted to suit your personal tastes.
The End. (much cheering and rejoicing)