C Drive is now F Drive

I just loaded winXP on a system that had its IDE ribbon and jumpers set in such a way that the BIOS saw the hard drive as the master but windows loaded the slave with C: and the hard drive with F:. I changed the slave (ZIP DRIVE) to G: so that C was available but since F: is a systems boot drive, I can’t change its letter. Is there an easy way to change the drive to C?