SInce this is a laptop if it is over a year old it could be that it has gotten dusty enough to raise the temperature inside to the point that the power supply will not be able to produce correct voltages which can cause the computer to lock-up.
Best way to get it fixed is clean it or have it cleaned, then get a cooling pad for underneath it.
Another possiblity is with the video. Your video card on this system uses shared memory, so the more processes running, the less memory is available for it to share. Only thing you can do for that is to turn off any processes not needed. Go to
http://www.blackviper.com/ and he has many processes listed that can be turned off with no adverse effects.