From what you describe, it's almost certainly not a browser fault. As IE and Firefox are giving you similar problems, I would suspect that the fault is within the operating system itself.
canard's advice is a good starting point. Clear out all the rubbish and make sure you don't have any unnecessary programs running.
What your describing does suggest something is using a lot of resources in terms of CPU usage and memory. XP does have a System Monitor utility, which will show you whether your CPU and/or memory is being hammered.
You don't mention the specification of your machine. Is it an older computer where perhaps XP has been "retrofitted" and is not really up to the task of running it? I mention this, because although I admit to not liking XP, it is normally relatively stable, and you suggest in your post that XP has been a problem to you.