If you can't access safe mode (Tap the F8 key as the machine boots)
You may have to try a reapir of XP.
Your dell may have come with a recovery partiton, but using this will cause loss of data and applications.
Go into your BIOS screen (Usually by tapping the DEL key on boot up.
Change the boot order to boot from DVD / CD first.
Follow these instructions EXACTLY: