Check that the air coming out of the laptop is not hotter than expected. It could be that one of your fans is having problems. That causes hardware hangs and freezes. Also make sure that vents are not blocked and air can flow freely.
Then there is memory checking. What version of windows do you have? For windowsxp you can use
Memtest86.
For windows7 especially 64 bit, you can do the following.
Go to start-Run (or if run is not visible, go to Start and type into the search programs window at the bottom)
Type mdsched.exe into the box (whichever one) and select to run the memcheck the next time you restart.
Restart the machine and check the report.
Once you are sure you don't have a hardware issue, you can start looking at drivers.