I had a similar problem like yours with Java, Sunione, that's why I mentioned downloading the newest Java, which worked for me. Perhaps this will work for you, not everything is virus related, there are other factors mentioned by Chris. A computer technician awhile back mentioned you have the biggest virus and that would be Windows, we both laughed and I knew exactly what he meant.

It may be corrupt.
Try removing it, restart the pc, then re-install from the web.

Your problem is not a browser problem.
I am sure any virus attacks with downloading the software.

My firefox crashes a lot but I'm sure it has something to do with FLASH. Flash is known to be very demanding on hardware resources. Sometimes my system lags if I allow the flash based game farmville to run in the background. Opening a site with a lot of embeded youtube also makes my PC crawl for a couple of seconds.