I ran it on my old Dell computer in 32 bit and it worked better then XP, faster and more reliable.
On my new Falcon Northwest computer in 64 bit it works even better.
I have had almost no problems with old software or drivers.
The two exceptions were...
Adobe Indesign CS2 would not run on 64 bit computer and I couldn't find drivers for my Epson scanner.
I upgraded Indesign to CS4 and found an app that runs my scanner so now I'm all set.
Even my old Tomb Raider games run fine, I really didn't think that they would.
My favorite thing in Windows 7 is the snap feature. It may seem like a little thing but it is a great time saver.
Just grab a window and drag it sideways until the cursor hits the edge of the screen and it reformats so that the window takes half the screen. I use this constantly to show 2 windows side by side on the screen. I can drag links from my E-Mail to my browser or Word whatever.
Windows 7 has been much more stable then XP for me.
I have been using it for about a year (I was a beta tester) and have had very good reliability.
My new computer has 8 gigs of ram so I had no choice about going to 64 bit.
If you are going to be installing it on an older computer you might want to go with the 32 bit version.
If you buy a retail copy you will get both disks.
This will avoid any driver problems, both Indesign CS2 and my scanner worked fine in 32 bit.
Windows 7 definitely. Every time I use Xp I find myself frustrated by what it can't do that Win7 can. I just love the new start menu. To open anything on my computer its just "WinKey+"Program Name">Enter" So easy. Multitasking is 100% smooth and Libraries are wonderful.