This other hard drive when you put it in did you start an operating system that was on it?? like you left the xp drive plugged in to copy files to it and the one you put in, did you start up whatever operating system was on it?? If so that's probably why. I've had that happen to me a few times myself.. XP is RELALY weird like that.. if you have xp and stick another drive in with say 95, 98, or ME on it and boot it, it does something to your xp drive.. If that's exactly what you did, then you probably need to reinstall xp. You can probably just do a repair. I don't remember the codes to do it but if you feel like doing some googling, look up how to use the recovery console to reinstall all the boot files. I don't remember what they are. :-( sorry but that'll save you from losing anything and anything getting messed up/apps not working, and drivers missing things like that. Give it a shot! it can't hurt since you won't lose anything doing it that way.. IF YOU DO do the repair, it will still do what I said but you'll have to redo your drivers most likely and reinstall some programs like antiviruses becase they wil be corrupt. Good luck. :-)
Jay