Believe it or not, the problem was with Firefox 3.5. I've been having huge problems with the latest version of Firefox and I've actually stopped using it now on my main computer. Touchwood, it seems to be okay on the computer in the bedroom. I'm now using Google chrome for my browsing, and Internet Explorer 8 for other things.
After posting on another computer forum that I use quite a lot, it does seem that I'm not the only person who has been having problems with the latest version of Firefox. Apparently some people have been having problems with Firefox 3.5 when using Vista. Since I'm on 64-bit system, this could well be my problem.