insert the OS CD , if Windows XP Reboot Computer
A message will say , Press any key to boot from CD...., Press enter , if not Restart and enter BIOS and go to Boot Section and select CDROM as the First Boot Device.
After you pressed enter , the setup program Loads a few files and takes you to the DOS welcome Screen , Press R to enter Recovery console
When at the command Prompt Press 1 to logon to C:\Windows
Enter Administrator Password if you have one.
If you see a C: Prompt , Enter FIXMBR and press enter , and Type in Exit or type in help if you not sure
Restart by Typing Exit
Remove the CD and everything should go fine