Intermittent Freeze Crash

we have had the computer in question for about 3 or four years now, however starting approximately a year ago it started to do this weird thing where randomly the entire computer will completely freeze, if there is any sound playing at the time of the freeze the sound loops repetitively and this will continue until the computer is reset.I tried to see if there was any specific cause for it doing this but i can only say that it happens more often during more intense programs (Eg: I-Tunes, video games, more demanding browsers like Chrome and Opera) but it will crash on occasion while doing nothing or running less intense programs like Internet explorer or paint. I have done numerous checks for virusesadwaremalware and nothing comes up that ultimately fixes the problem when it is removed. I have done several scans for registry errors and still nothing fixes the problem, at one point i even used the windows boot disc to delete the registry and restore it back to default, which as you can guess did not fix the problem and mainly just left me with a huge headache.I have reset the Bios and that doesn't work. i found something relatively similar to the problem i was having which would suggest that my RAM is bad so i ran memtest but after 4 passes it came up with nothing.I used the vacuum cleaner to clean out the whole case and still no results.