Windows 7 Windows 7 Blue Screen Of Death with error: IRQL_NOT_LESS_OR_EQUAL

Hello, everyone

This happens frequent 4 times or 5 times a week

I use my desktop windows 7 64 bit (OS 3 years old) mostly for browsing, and I use Firefox mainly for browsing what happens Firefox crashes and then a message pops up as below :

(http://imgur.com/a/bc1YI0Y)
And then blue screen of death with error: IRQL_NOT_LESS_OR_EQUAL.

(http://imgur.com/a/MkusGGI)
Sometime even not browsing if I open a pdf document also that crashes the system and the same blue screen of death with error IRQL_NOT_LESS_OR_EQUAL pops up

Any troubleshoot should I do to fix the issue ?

Thanks