I personally use LogMeIn a lot. Their free option is excellent for normal usage. You can control a PC or Mac cross-platform right in your browser. It doesn't allow you to transfer file between two computers though, I'd suggest use Dropbox with LogMeIn, if you need this specific function.
I've got a Mac Mini running XP (Long story short, it's got a huge cut across the memory pins, and OS X wont install because of it. XP works fine though, so was the only choice) for a HTPC.
From my iMac and MBP I login to it using the Windows Remote Desktop client for the Mac.
I'm using it on XP Pro. I dont think it would work on XP Home.
Another alternative, just use a VNC client/server. Chicken of The VNC and any windows VNC server would do it