unmountable boot volume?

I think I might have a pretty bad problem:

A few years ago a friend gave me his computer, and when he gave it to me he switched from windows 98 to windows XP, so I don't have the XP cd.
After having the computer for about a year, I got an error message when I tried to turn it on, saying that windows didn't start successfully and mentioning that a recent software or hardware problem may have caused it. It also lists some options like safe mode and start windows. No matter which option I select, I get another error message after the windows load screen, this time it's blue and it says something about UNMOUNTABLE_BOOT_VOLUME, and lists a few ways to fix the problem (all of which I've tried.)
My next step was calling Dell, but they wouldn't give me much help after finding out where my OS came from, saying they don't techsupport third party software.
So the problem is I can't start windows, I don't have the XP disc, it's a really old computer, and I don't have a vendor to whom I can complain.
Please help?

Have you done any hardware upgrades to this computer lately?

I haven't used the computer in a while, but not that I recall. I think I'd just gotten my webcam, but it was working fine after I installed it.

Go here and try Microsofts' solutions, but I suspect you are going to need to reload XP. Good luck.

Crap. this sucks.