as soon as it's turned on it will go through loading and then it gives a # 4 error message.It also says it can't find my Os.I don't know what happened to it .I was using it one time and the next time I turned it on it was like this.
It may not be able to mount the hard disk properly and the first step is to run Windows Checkdisk to try to repair bad sectors. If you still have your XP CD, pop that in and set the BIOS to boot from CD. At the Repair option, select that and when you see the Command Prompt, type chkdsk /r (including the space) and that will start a five stage process which might recover the disk to a usable state.
If you don't have the CD, you might like to take it to a fixer who will have the facility to put the hard disk into a converter and connect it to another PC to run that test, or you could buy a converter on e-Bay and do it yourself.