I have two purchased copies of XP. I’ve moved them around from computer to computer and registered them each time. 7 is nice, but XP works just fine for me.
Vista was crap, of course.
Have it on my laptop. Don't use it much anymore, but it'll work in a pinch.
One thing I've found about Windows 7, which is on the computer I'm using is, MSN Explorer doesn't work worth a hill of beans with it. A Microsoft product won't work with a Microsoft product.