Windows generally does just end up becoming slower as programs and updates go in and out of the registry etc so as said a rebuild of the software usually sorts things out.

By working hard, do you mean you can hear it making any iffy noises? As in crunching or a repeated clunk noise? If so that would suggest HDD. You could do with some more ram to speed things up as well. Testing it is quite easy if you have other RAM to hand or a spare PC lol - Otherwise there is software you can download to check it.

Failing that if it's something that's happened fairly recently you could try using system restore to put it back before that time. If it works great again you know it was an update or something you installed during that time.

HTH