There is NO situation (obscure raid configs aside) where you can't take a hard drive from one machine and put it into another and have it boot to desktop. Anyone who says different doesn't know what they're talking about. The only reason you'll not get to desktop is lack of storage controller drivers. There are 3 ways to do it:
1. Whilst the machine is up and running, prepare it for the move with the relevant drivers.
2. Throw the drive in the new machine, then run a repair install on it from the CD to add the storage drivers.
3. Boot the new machine from a Windows PE disc (google - universal boot CD for windows), then manually add the required drivers and tweak the registry hives on the hard disc as appropriate.
All the above require at least a moderate level of Windoze internals knowledge, but all are perfectly possible. If you don't have the inclination to get stuck in and make it happen, reformat and as Keef says, slave your old drive. If you do, google is your friend.
BTW. Windows sucks donkey balls, have you considered Linux? I used to begrudgingly assist my friends with their PC problems, these days I just tell them to buy a Mac and spend my spare time flying instead.
Cheers,
Shunter
Ashamed MCSE, MVP and former Microsoft trainer.