Empty all your temp files. Get rid of any of those stupid effing Yahoo and Google toolbars if you have any.
Do a defrag.
Run an up to date anti-spyware app such as
http://www.safer-networking.org/en/download/index.html
Don't go to spybot.com or any other site than the one above. There are many bogus sites selling spyware pretending to be a Spybot site.
What sort of anti-virus software are you using? Norton, McAfee and some others can sometimes slow your system and cause locks and hangs.
Re-installing your browser can sometimes help (if you're using Opera, Chrome or Firefox...).
OR, if it happens on big threads with lots of images and YouTube links, you may have a memory problem. Possibly some dicky RAM that gets accessed while loading all those images or something...