Hi
Dam, your in trouble then pal, do you know anyone with a disk that is the same as the OS (there is XP Home and XP Pro) if so ask if you can borrow a copy,
For now turn on the PC and look for the message "Press X (whereX is a key) to enter setup or BIOS.
Go into BIOS and look for anything that shows the harddrive, it may be under the section "IDE Devices" or similar, if it shows up then the HD maybe OK, if it doesnt then the drive may be dead, in which case you may need a new drive.
Looking at the wider picture it may be best to take the PC to a shop, it depends on how savvy you are, the things you are going to need to do are..
Download the HD manufacturers test tools to test the drive (only applies if the drive shows in BIOS)
Obtain a copy of XP and if the drive is OK do an repair of XP
If the drive isnt showing in BIOS then it may need changing, it's simple to do if your OK with opening up the PC
Post back and rate your self on a 1 to 10 scale (where 1 is a totally PC unsure and 10 is a top rate IT technician) be honest, we wouldnt want you to do something that you may get wrong and cause further problems.
Regards