First of all, are you using UT's OpenGL renderer? If you are, I suggest you switch to D3D quickly, because it says right in the program that OpenGL isn't officially supported. On my computer it causes a memory leak when the game closes.

Second of all, is your mouse a USB mouse? I imagine that could cause some sort of trouble with a computer that is as old as yours. Did you install software that came with your mouse? Whatever you do, do NOT install software that comes with your mouse. All that it is is an irritant.

At first, I thought you were getting boot codes when you said you were getting beeping noises. But it seems you are able to boot. It very well could be a virus, but it sounds more like old system instability to me. Too much crap installed on an ancient OS, I assume. Or a new OS installed over an old one.

I think ultimately you should either buy a new computer, or back up all your crucial files, and then reformat your drive, and reinstall Windows. That always does the trick.