Control Panel > Administrative Tools > Event Viewer
Or on the Run dialog just use: eventvwr
And just because a diagnostic program says there are no RAM errors doesn't mean the RAM isn't causing the problem. This could be caused by RAM, video card, hard drive failing, or PSU, those being the most likely to choose from. You should test the hard drive with the manufacturer's utility, try the system with only one RAM module installed, and try it in all the slots one at a time (time consuming of course if it's an intermittent failure).