ACID this sounds like a hardware problem (why would you even ask Regulus if tis soft/hard how should he know!!!) What probably is happening is your PC is overheating and shutting down. Thats why your computer will shutdown at random times, usually around CPU critical times. What you should do is buy a can of air from Best Buy, open up your pc and air out all the dust thats clogging. All the dust is making it overheat so get rid of it. As far as the beeps go it is probably your internal speaker on your motherboard. Every time you turn on your PC it runs something called POST to make sure everything is running fine. If there is an error the internal will give different amount of beeps to indicate different problems. Im not quite sure what 3 beeps mean but check out this site http://www.hi-tech.net/zones/help-beep.html
and make sure you match up your type of motherboard BIOS on it (you can find the bios type usually when you first turn on your PC) If the dust is the problem tho you wont have to worry about the beeps so try that first.