I have an annoying problem. I am thinking virus, but AVG is up to date and not picking one up.

My internet explorer runs fantastic when i connect, fast as anything until ive been browsing for approx 30-40 mins. Then slowly it stops finding websites, i get the ''Internet Explorer cannot display the webpage''. So i try to close exploer but it wont and goes into 'not responding' mode. Only way to close is to keep pressing and pressing close in the task bar(ctrl, alt, del). But then once closed it wont open properly again, even when i disconnect from the net and reconnect. Only way to get back on the net is to turn off the pc and start again, and after 30-40 mins it happens again...and on and on!!!

Whilsit this is happening the hard drive is buzzing like mad....

Any ideas?