If your bios has SmartD support, reboot and go into that and see if it reports errors. XP may also let you see the SmartD log if you have it enabled -- I haven't used XP for more than running Windows Update, so, I don't know if the utilities are built in or need to be installed.
To me, it sounds like you are having sector read/write problems and that whichever drive is there is left to fail.
To me, for a computer that you rely on for business, Raid-1 is a very inexpensive way to help protect from some disasters. If both drives are in a Raid-1 Group, and one drive fails for whatever reason, with Raid-1 you've got your data mirrored on the other drive. Case in point though, as my wife found out, a boot sector virus is faithfully mirrored on the second drive in the set.
Raid is not a substitute for backups though -- a stolen or burned computer with Raid-1 drives provides you no protection.
Make a backup. It is possible that it is only a problem with some of the media on the disk and if you can figure out which files are the problem, make a copy, leave the files there, rename the directory and don't delete it. Or, buy Spinrite which will go through and test the drive and move things around if it deems it so.