Both XP and 7 were revisions of previous OS's.
Windows 2k was 5.0 (decent)
Windows XP was 5.1 (awesome)
Windows Vista was 6.0 (decent)
Windows 7 was 6.1 (awesome)
Widnows 8 will be 7.0 (so far pretty bad)
It seems that when Microsoft makes a fairly major change to the OS the first version isn't all the spectacular but when the go back and revise it with the next version it's pretty good. For that reason alone I'll probably save my money and not buy Windows 8 because all I really use Windows for now is gaming, and I stick to UNIX based OS's for everything else.