|Basically, dependent on how old the PC is, you could honestly have a motherboard issue with your PC. If you are not hearing BIOS beeps indicating bad RAM, then it is not an issue with the RAM, but rather with the motherboard. Or, on a secondary note, if the RAM was freshly upgraded and installed, pop the old RAM back in and see what happens, if nothing else.|
If it is after the splash screen, it could be a Windows(r) issue. See if you can press F8 and try Safe Mode, if a software problem.
However... from the symptoms described and from computer experience, it sounds like a board or RAM issue.