The fact you installed a new stick of RAM worries me slightly.
Is it of the same spec/model as the other RAM modules in your system at the moment?
That won't cause a massive problem, or at least shouldn't, but compatabilities very important for stability, and if for example you are running 2 different makes, that may have 2 different sets of timings; this could cause some problems for the system.
If the RAM module is the same as the rest however, then you could have got DOA (dead on arrival) RAM, and may need to be sent back.
There are checks you can do to test it though, so report back
Ah, I'll tell you now:
Go here, and download RAM Probe:
Here, download the ISO, and then write this as an Image file in for example Nero, or any other burning software you may have onto a standard CD-R disc.
Restart the computer, with the BIOS set to run from the DVD drive first, and let the program perform the tests on the RAM.
It may be best to take the other 2GB's out, leaving the new one in, and let it run. If it finds any problems, you know right away there is an issue.