Bold font problem in web browsers

I've had this problem for a while now and it's really starting to annoy me! On some websites the text goes bold, the problem used to only appear occasionally but now it happens all the time (see attachments for example). It happens in Safari, Chrome and Firefox so must be an os x issue? I'm running Snow Leopard 10.6.8

Thought I'd try here and see if anyone had any solutions before I go down the route of re-installing os x. Thanks!