Internet explorer and Firefox not opening

Hello,
my brothers have abused my computer to the point that internet explorer and firefox wont open. All I keep getting is the error message
'the memory could not be writen' on firefox and on internet explorer but I can load netscape

I have searched under run and have check the key Shell and Userinit, neither of the values have anything else except for what is meant to be on there.

I did a virus scan using avg and it hasn't picked anything up.

What else can I do?

p.s I have uninstalled and reinstalled firefox to no luck.

I'm using vista as well.