Well.... technically it was supported until 2013 now(they keep moving back), however it will be Win 7 after that I'd be shocked if they continue supporting it longer.
It really depends on your hardware.
Win 7 requires better video for Aero features(pretty interface effects)
--Has more drivers included on dvd/out of box
--Printers install easier
--It just works
--64bit is iffy but needed for large amounts of ram
--support will expire in 3 years
---some newer devices/manufactorers already stopped guaranteeing compatibility
If you told us what you wanted to use it for and what system you were putting it on then we could better answer.
Personally unless it's 3+ years old I'd go with Win 7.