Strange Problem

Could be whats called 'memory leak', whereby when you start a program, close it, then start another program (eg your media player), the first program has not released the memory it was using. This obviously causes the free RAM to progressively drop to such a low level that Windows then has to resort to the much slower 'virtual memory' (on the hard disk). This causes delays where the computer is playing 'catch-up' and you get poor music playback and generally poor response.

One sure fire way of getting back all the free memory that should be available is to re-boot -- but obviously that's annoying as well as inconvenient. A better way is to use a 'memory optimiser' which forces all the 'leaked' memory back in to the memory pool without having to re-boot. There's a shareware one here which looks dead easy to use:
http://www.popdrops.com/freeupram/

Freemem Standard is another good memory freeing program and is free.

http://www.meikel.co.../comparison.php