What does it mean when your system restarts for no apparent reason, or when you experience frequent program crashes and file corruption? The problem might not be with Windows at all. These baffling symptoms can be the result of a bad memory chip.
If you suspect you might have a failing memory module, Microsoft has a free downloadable memory test program that you can use:
The Windows Memory Diagnostic tests the Random Access Memory (RAM) on your computer for errors. The diagnostic includes a comprehensive set of memory tests. If you are experiencing problems while running Windows, you can use the diagnostic to determine whether the problems are caused by failing hardware, such as RAM or the memory system of your motherboard. Windows Memory Diagnostic is designed to be easy and fast. On most configurations, you can download the diagnostic, read the documentation, run the test and complete the first test pass in less than 30 minutes.
To run Windows Memory Diagnostic, you must reboot your computer with the disk or CD-ROM on which you installed Windows Memory Diagnostic in the drive. After the reboot, Windows Memory Diagnostic will load and its interface will appear. After loading, the first test pass will begin, using the default standard test suite, and continue until complete, unless Windows Memory Diagnostic is either paused or exited. Once the first test pass is complete, Windows Memory Diagnostic will begin a second test pass using the same settings as before. Windows Memory Diagnostic will continue to run test passes until you exit.
If you find a problem, try pulling the suspected memory chip and running with reduced memory for a short time. If the problems vanish, replace the memory.
Ed Bott 
This little tool definitely did the trick for me. Found errors on one of my RAM chips that was causing random reboots and was very easy to run. Bought myself (2) 512mb chips and installed – reran the test – all is GOOD! Definitely worth having in the arsenal.
Mark