Why does my internet browser keep closing?

[LovelyLotus]

A relative of mine gave me their old laptop, I'm only going to be using it until I get enough money to buy a better laptop but in the meantime there I things I have to get done using the internet. My relative told me that there used to be virus's on the computer, I scanned the laptop and did find some virus's. So I used an anti-virus software to remove the virus's, now it says there gone, but I'm still having problems.

After about ten minutes my internet browser will shut down, without any warning or messages. I've tried using internet explorer, Google Chrome, and now I'm using Firefox. I've already gone through my add-ons, redownloaded my flash player, adjusted the firewall, reset the internet, and deleted everything from the computer and it's still doing it. I don't know what else to try anymore.
Is there any hope for this laptop? Any ideas what it can be and how I can fix this?

Thank you