I rebuilt my computer about six months ago. I was running XP Pro. I installed Windows 7 Ultimate (which was gifted to me) and I haven't looked back. I am pleased with several things :
1. Its self diagnostics seem to work - and I put them to the test until I got all of my hardware right.
2. Very stable
3. The folks at M$ have done a remarkable job with the services. I never ran Vista so I don't know what Vista's services were like. But Win 7's services are a dream compared to XP's. Most service are set to manual and only start if needed and the OS does a good job of stoppin them after they are no longer needed.
I still have an XP machine; but I recommend Windows 7 if your computer is relatively up to date.
I should add the OS is locked down pretty tight and Administrators do not have the free reign I enjoy in XP.