LOL well if it's IDE just make it the slave. If it's SATA see if you have hot-swap capabilities. The only thing you have to worry about is it booting the bad drive as primary, because then it's just going to screw up (wrong hardware, messed up OS, etc). You pop the drive it, go to BIOS, check the drive settings (I know on this machine any time I put a second drive in it re-orders my boot), make sure you're booting to the proper drive, then you're good.
As XP loads it'll load from the good drive. It won't even read the other drive until it needs to see how many drives to display in Explorer. That's my experience on the matter.
I don't know about infected drives, but I've done this on more than one HD that has failed. It doesn't corrupt the good drive. If the drive is actually bad, you won't be able to complete a full scandisk on it, or copy files from it. Just burn backups for all the important files on the drive, wipe it (format it), and do 1 of 2 things
If you can't even format it: dump the drive, buy a new one, restart with fresh install
If you can format it: pop it back in the original machine and install Windows again fresh.