i take it that you just pulled the Hard Drive out and put it into the new shuttle? if its a different motherboard it wont boot because it will still be trying to load the drivers for that specific board
so yes...your only real option is to do a repair install of windows
this is done by booting from the disk...which you cant do
so...did u change any jumpers? because I know on some motherboards there are jumpers u can change to disable the PS/2 ports and stuff...can you access the BIOS and move around?
If you do, by some miracle (perhaps try another keyboard?) get into XP setup, you want to skip the first screen untill it says scanning for previous versions of microsoft windows, and it should detect ur XP setup, and then just click r on the keyboard to start a repair install which will keep all your desktop setting and the like but set it up for the new motherboard
Goodluck!
Kurtis