Generally you would boot into your raid controllers bios and add the drive back to the array. Depending on the controller though, there may be software installed within Windows that would allow you to do so. If you got an error message in Windows likely you have some software within XP that will allow you to rebuild the array. However, there is probably a reason a drive dropped out of the array. I would try running the manufactures drive test on the drive that was dropped to determine if it's bad.
If you have IT support call them, and tell them to come fix your machine. It's not worth hassling with if you have people who are paid to sort that out for you.
Backing up the current drive is also a good idea, as I'm guessing the Raid 1 set was supposed to serve as the backup.