I still run Windows XP SP2 at home and at work. I'm pretty happy with the performance of the OS on my three year old hardware and it does all the things I expect from an OS.
I would upgrade, but Vista seems too unwieldy and has many feature - which I may one day find useful - but they are certainly not essential. I'd upgrade more frequently if Windows was simpler. My ideal Windows upgrade would:
1. Be easier to upgrade.
2. Run on my existing hardware
3. Componentised so I can choose which features to install, to improve security, stability and simplify patch management
4. Licensed so that I only pay for the functionality I require
5. Remove Internet Explorer and Windows Media Player from the core OS