Let me start by saying that your post is unclear on a number of points: first, which machine are you now trying to use the drive in -- the original, or the other one? Second, did you try to use it as the boot drive in the other computer, or did you just put it in as a slave so you could copy files over?
In most cases, you can't move an XP or Vista boot drive to another system to use as the boot drive there because, first of all, all the drivers will be wrong, and second of all, Microsoft calculates a value based on your hardware that has to match up to the installed OS or else it assumes it isn't in the machine it was licensed for, making the license invalid.
That said, if you have the original XP or Vista CD's, you can do a repair install in your original machine. If you only have restore CD's or a restore partition, then you may (or may not) be able to do a non-destructive system recovery. Either a repair install or a non-destructive system recovery would leave most of your data intact, but you might lose your favorites and all your e-mail (assuming you were using Outlook Express) and address book.
So, if you're not sure, and you don't have a good backup, you better get your drive hooked up as a second drive on some other PC and save your data!