^ Yes it is your computer dude but Microsoft release updated browsers (for free obviously) so as to keep up to date and fix issues so where is the sense in not updating and no, they don't force people to upgrade, it is a choice we have.
I am currently running the IE8 Beta and it seems pretty stable.
I have used FF and seen that thing completely fall apart with the issues they had within some of the version 2's.
I have used IE and seen the resource hog it is and also crash.
I still use IE (v

FF (v3) and Safari, though my current browser of choice is Safari (for the time being lol!)
One must also remember that as IE has the lions share of the market then almost all hackers etc will write hacks to attack this and if FF had the lions share then they would switch to this..