I purchased a Penryn MacBook in March. It was my first Mac and I'm loving it right now, so much better then a PC. I bought the low end MacBook and it's faster then my PC is, PC has a slower CPU (not by much) and 2GB of ram. Now, I don't think they will be updating the MacBook's soon because Montevina has been delayed till July 15th I believe. Right now though the MacBook is a great buy, I still gotta get AppleCare for mine but I have till March so that's all good.
If your Mac is faster than your PC then there is something wrong with your PC. Leopard is slow on 1GB of memory (I just upgraded to 2.5GB from 1, huge difference).
However I have a Mac and a PC. Mid-2007 MacBook w/SuperDrive and 2.5GB of RAM. HP with 2GB of RAM, 2GHz C2D (Santa Rosa), GeForce 8400M GS.
My HP is faster when it comes to CPU intensive tasks, it can play games, it can play high definition movies, it actually has a GPU that can be taken advantage of for video playback, HDMI output, TV tuner, etc.
All around, my PC is way better than my Mac.
You're just still in the honeymoon phase and you haven't seen what hardware you could have bought for the same price (think $2,500 MBP for around $1,000).
Anyway, to the OP, Vista is a little slow at first. But the more she uses it the faster it will get because it will cache data and such relevant to what she does. After a week or so her Vista machine will be faster than any equally spec'ed Mac. She probably spent half as much on that Dell as she would have on a MacBook too.
Don't expect the MacBooks to be problem free either. She'll still have to install Windows to get full functionality. She'll also have to worry about endless build quality issues too. Peeling, separation, cracking due to heat or otherwise, HEAT, discolortion (yes it still exists). The MacBook panels (look up the specs if you don't believe me) also have painfully slow response times of 24ms. You get ghosting just moving the cursor.
Shes much better off with the Dell and she'll save money in the process. Just uninstall the crapware (which she could have deselected when she was configuring the PC for purchase) and run ccleaner. It'll run just as good as a fresh install.