I work with computers all day, so this would be my recomendation. It sounds like you have gotten a registry virus as suggested before. At first I thought maybe your hard-drive just crashed but you are able to boot up so this isnt the case. Doing a reg-edit on your computer would not be recommended either unless you have the exact script of what to take out. Messing with the registry can be fatal to you whole system, as also flashing your bios. If you take out one extra period needed for the OS your system will never boot again. My suggestion to you is buy a wipe program and go through the walk through process of writing zeros to the drive, to be on the safe side wipe the disk clean 3 times in a row, this will hopefully wipe everything clean, including viruses embedded into the drive. next try to reload your OS (windows xp) with a clean install meaning from scratch, formatting, loading, etc. Hopefully this will work and you will regain access to your computer. But at a last resort if this doesnt work then you will have to go purchase a new hard-drive, format it and install you OS. Once you get your computer working again, I would also suggest setting up, a anti-virus program, a firewall, spy-ware remover, spam-blocker, ad-ware remover, (all of these are free from downloads.com) except the anti-virus software. the firewall being the best protection ( I suggest zone alarm). All these tools will not prevent all viruses from your computer but it will certainly help out alot more. Viruses are what keep people in business, without them there would be no need for those expense tools (thats why i suggest the free ones). They are a pain, and I know what you are going through, Ive had them crash my hard-drive and had to start from stratch, also. hope this helps
just when you think you know everything about computers they come out with something new