After you key in "R", type "fixboot C:". If the Vista root is not located on C drive, replace it with the respective drive. Then input the command "fixmbr C:" and enter "EXIT". Next, close the setup program and restart the computer. Make sure that the MS-DOS boot CD is still inserted in the CD-ROM drive. Then reformat your hard drive by typing in "format C:". Once the reformatting is complete, put in the Windows XP CD into the drive and restart your computer. It may seem complicated to uninstall Windows Vista but when the XP installation menu emerges, you can easily follow the instructions to install Windows XP onto your computer.