HP Probook 4320s - Frequent Crashes / Freezing Randomly

My HP Probook 4320s laptop was purchased in October 2011 - It quickly developed the extremely annoying problem of freezing / crashing. I called the company that it was purchased it from, they asked me to do a memory test & hard drive test - they showed no errors. They then advised me to reinstall Windows 7 Home Premium 32bit (it was preinstalled when purchased). I reinstalled it, & downloaded all of the drivers from the web (a painfully slow experience on Kenyan internet...!!). After installing all of them - it's still happening! Sometimes if left for a couple of minutes it will start to work again, sometimes it needs a reset with the power button.