In reply to Chief1, that's a good suggestion, but I think it is highly unlikely on a home system, especially one that has just been purchased. The problem only affected you if you were running certain 'non standard' applications, i.e. something other than bog standard MS software or games.
You won't be doing much paging if you have 512Mb of RAM, whereas you may well be with 256Mb RAM or less - check 'virtual memory size' under WinXP help for MS recommendations as to how to set/alter your page file.
From a very crude basic point of view, you can get an idea if your PC is struggling, if you keep seeing the red HDD light flashing when you are running something. Normally, once you have loaded your application (or game or whatever), you would expect most of that code to be kept in RAM, so the only HDD access should be for reading/writing data that your application needs.
My money is on a some malware, although if you are trying to run too much on this box, then you could be suffering from 'thrashing' i.e. excessive paging activity as WinXP tries to manage the inadequate ammount of RAM it has, by paging bits of it to and from your HDD all the time.