If you require any of the files take them off the drive then follow these instuctions;
Reboot with WinXP in CD-Rom
WinXP should start installing
When it comes to installing select your current drive delete the partition and the remake the partition and then it will install XP onto that drive (full format not quick)
Note: Partition bit v easy and has onscreen instructions, if you have more than 1 HDD disconnect the other one(s) so you dont accidently format that one by accident