Hi
The first thing I would do is to open the C:\ prompt as administrator that type SFC /scannow.
It will search for and repair any bad system files.
You may just be running out of memory, 2 GB isn't much anymore.
Most new computers come with at least 8, and many have 16, I have 32.
Your computer could be overheating, have you cleaned it out lately?
You can check the temp using HWMonitor...
http://www.cpuid.com/softwares/hwmonitor.html
Look at the temperature after a half hour and then at 2 hours or about the time it goes off and see it they have gone up a lot.
How long has it been doing this?
Do you have a restore point from before that?
Any new software added?
Run CCleaner...
Link Removed
That's all the easy stuff that I can think of, it's possible that you have some component failing or overheating.
That's hard to guess at, the fact that it works for a certain amount of time then fails sounds like heat to me.
But it's only a guess, if none of these ideas help, come back and maybe someone else will have an idea.
Mike