As fo3nix says: most of the code is the same as XP. Most of the 5 years was spent dropping things from the original spec, and adding all those "Do you wish to continue?" pop-ups. The only real re-write is in the graphics engine.
That said, it refused to recognise my motherboard PCI bridge, Epson R200 printer, Epson 2480 photo scanner and Fritz ISDN terminal adaptor.
It wouldn't run Delphi 7, which hung it so badly, that the only recovery was by means of the mains switch (to be fair, it did warn me that Delphi was not compatible).
Lots of my trialware programs no longer recognized their licences.
On at least 5 occasions it booted to the login screen but showed no task bar or icons.
Still - all worth it, cos now I can see through the borders of the windows! (That's sarcasm, BTW).
Thank God that (a) MS supplied it to me for free, and (b) I installed it on a spare hard disk (which is now back in the cupboard)