Ok. It's time to give up and go to a service center. I wrote a whole long spiel about how to recover your computer and your files before I saw that you can't even boot in safe mode.
You're jamming F8 repeatedly and still can't hit safe mode, yes? Because safe mode would let you do a LOT that you can't do right now.
It may be time to give up and go to a service center. Any service center worth its salt can pull the hard drive and back up your files from it. It's pricey, so decide how much that stuff is worth to you. At this point, if you really can't get into safe mode, it's your only reasonable option short of having a buddy who works in IT and really works in IT: not a cousin who's "real good with computers." Those are to be avoided at all costs.
Got your data backed up, or decided it wasn't worth $100-$200 after all? Good. Step two: ignore the service center's urging that you need to let them fix it right this very second. Contact HP and tell them you need to buy the recovery disks. Should cost you between $30 and $40.
Wait. They'll ship them to you.
Now, boot from the recovery disks. Follow the instructions. If all goes well and you don't have any hardware failure, you should be back to factory settings. Reload all the software you had on there, and dump the backups back on. Oh, and go out and buy some really freaking good antivirus and anti spyware, and put that on before you let your computer talk to the interwebs. No exceptions! Update them and update windows at the first available opportunity.
If you aren't sure which antivirus software is good, ask the IT guys who were trying to sell you the $300 virus cleanup. Trust me, they know.
If for any reason the recovery fails, you're back to the service center. Tell them what you use your computer for and how much of your software only runs on XP, and ask them whether it's worth repairing your machine or if you need to buy a new machine.
And when you get that new machine, or they repair the old one, back everything up and get some great anti-malware on there. Set it and windows to update automatically and frequently!
Good luck, and tech bless.