System Resources issue.. Help?

[styla21]

Wondering if you guys can help out on a random comp problem I'm having

My programs are consuming way more disk space than they should be!
I've just installed, uninstalled, then reinstalled the latest versions of Thunderbird, and Mozilla. Thunderbird, for example, should be using 24mb. It is instead using over 1600mb

What could cause this?

Also, task manger shows Mozilla 3.0 as using 51mb of system resources, even when displaying a blank page. When displaying MW it's around the 90mb mark.
I have run two spyware cleaners and full Avast anti-virus clean. Registry was just defragged. Any suggestions or advice appreciated!