Why does my computer crash so much?

My computer will crash at least 20 times in 1 day and I can not work out why. It is coming up to 3 years old but it has no viruses, i keep all spyware and virus scanning software up to date and scan regularly, I defrag regularly, I run CCleaner regularly deleting temporary files. I dont save my work on my laptop, I have an external drive for that, I barely move my laptop from where it is so it has not been bashed around at all and I regularly scan for updates for all my drivers. I treat my laptop as well as I can, letting it take its time over things and not clicking too much but it will still crash with internet explorer unable to stay open for more than an hour without an error message coming up saying that it is not responding even if i havent clicked on anything and am just sat with a pdf file open reading a scientific paper.
what am I doing wrong?
I have tried to use firefox but it stopped my 4od from working for some reason so i got rid of it.
I cant afford to buy a new laptop or pay for someone else to take a look at it. It is making me take about twice as long as I need to to finish research.