In Win7, just below "all programs", type memory in there, and run the windows memory diagnostic test. If that shows a problem, you can determine which stick it is from the results.
If it shows no problem, open your machine, ensure all sticks are securely seated. After that, its time to take memory out, put them in in whichever configuration your mb supports, and see if you can find any that are bad.
Hth,
Wurzel (you should hear beeps if your ram stick is bad tho, lots and lots of them)