My computer always freezes?

Since a couple months ago, my computer has started to freeze more and more often (every 20 minutes or an hour). The mouse just stops moving, and i need to turn it off by holding down the power button, which i know is bad for the computer. This seems to happen a lot more often when im on the internet, but i do virus scans really often, and my computer always comes up clean (Microsoft Security Essentials). Do you guys have any advice as to how i can fix this? I can't send it to a repairman because im on vacation right now. Its a Gateway FX 6831.