Can't Start Computer!

I have a severe problem. Every time I try to turn on my computer, it gives me an error message saying my computer did not start correctly, then it gives me several choices, like Safe Mode, Last Good Configuration, Start Windows Normally, etc. However, no matter what I choose, it passes the Windows XP loading screen, then the screen freezes and goes black. It stays like this until I reboot my computer. I'm pretty sure some important files were corrupted. I had similar problems a few days ago, when I could still use my computer. It said some files were missing, and somehow, both my firewall and Norton Antivirus were deleted. It told me to reinstall them, but after I did, it said there were corrupted files. After this, I rebooted my computer, and, well...I couldn't turn it on since. Right now, I'm using my friend's laptop. So, how did this happen? How, if even possible, can it be fixed without re-formatting my hard drive, re-installing Windows XP, etc.?

 

probably a virus especially if a firewall and an antivirus program got deleted... (unless someone went on your computer and did it)

if it was a virus I think you'd have to reformat