rojash is quite right, although if you're having major probs with your pc already, then the xpdisk may not give you the option to delete the active partition.
If you make an ms-dos boot floppy disk, takes about 5 mins ( just type ms-disk in your pc help, it'll tell you how ), and restart your pc with the floppy and the xpdisk in, and then choose the option to "start with cdrom support ".
then type "fdisk" , you can delete the partition, usually c drive, and recreate it again.
then remove the floppy and just restart and install your xp.
this works because, it re-loads an uncurrupted set of basic drivers for the your pc to work with.(lol i think)
hope you fair well :-)