I dont have pro i have home, but i found the easiest way for me is to reboot into DOS (useing DOS off a floppy) then when it askes how u want to start hit "2 (start with CD rom support)" then let it load the drivers and stuff in for the drives, when it gets to the "A" prompt ( A_ ) type in "FORMAT C:" it will ask if your sure and all data will be erased from that drive hit "Y" to continue.
The drive will be formatted clean,next i reset my system so it reboots, i put the OS disc in the drive then when you get to the A prompt again ( A_ ) type in depending on which drive you put the disc in "E: or D:SETUP.EXE" then it will run through the setup program for the OS.
I have to instal ME first then upgrade to XP, im not sure on how pro does it, i hope this helps bro.
peace