if your machine has just slowed down ... then either you have started doing something different or it's just filled up with binary fluff (temp files photos, music and stuff)
music, film and photo editing use huge amounts of memory - so if you've just started a new hobby it could be that - and you probably need more ram
BUT try a good clean up BEFORE you spent any money - it
WILL make an improvement - how much is not certain until you do it and see
this will make it easier.
http://www.ccleaner.com/download (it's free)
download, install, just press the button and it will do the rest
then run the window's disk checker and finally defrag
(both in disk | properties | tools)
(it possibly won't make a huge difference - but it's worth a try (and it costs nowt!) ... you should do it at least a couple of times a year (but don't go mad).
then if no difference ... it's time to spend some money