Hello, it is possible that the xp deframentation program messed up the vista MBR/MFT. You might try to repair the vista install using the vista installation dvd/cd. First boot the computer using the Vista CD/DVD, once you get the vista boot screen, select your environmental variables "laguage/time/keyboard" then click next. Then from the next screen select "repair your computer" you should then get a screen that shows the opperating systems currently installed; find and select the vista opperating system/drive, then click next. On the next screen select "command prompt"
From the command prompt type bootrec.exe /fixmbr then press enter, then type bootrec.exe /fixboot then press enter. From there, remove the vista installation CD/DVD and reboot your PC. If the XP defrag tool didn't do too much damage to the vista drive/system files, all should be well.