Laptop frequently freezes for no reason. AddedRAM but nothing happened

I recently (~6 months ago) got an Acer 5551 Laptop (2.1Ghz Dual Core, 2GB of RAM recently upgraded to 4) and it randomly freezes like nobody's business. I could have only firefox open with one tab and it will freeze with no other programs running. I have been running CCleaner to clean up the registry and get rid of extra junk but it doesn't help. There are no viruses on the computer (I use Windows Defender) and adding an extra 2GB of ram didn't do anything. It's windows 7 32 bit and the gpu only uses like .60 Gigs of Ram so I have in reality a little over 3GB, but that should still be enough to make it run smooth.. Yet there has been literally no change in performance. My last system restore point is like 2 weeks ago and it's been doing this for months. Does anybody have any idea what could be wrong? It happens all the time randomly-when it just starts up and when it's been on for a while (hot or cold.) I just don't get it. Any suggestions would be helpful.