If every time he tries to install some software he gets a crash (more detail on this would be nice, of course), with something as well-supported as EZDrummer, I'd say the biggest problem there would be software. Random reboots could be hardware or software, granted.
As for the home build thing... it's so easy now it would take work to get cooling wrong. Case fans, PSU and CPU fans... unless he's leaving it in his microwave or surrounding it with blankets there are bigger issues at stake. The main point I was making is that store-bought computers are in my experience just as bad as, if not worse than, home builds in terms of cooling.
Jeff