Hi, and Welcome to the best Forum on the 'net.
As you are using Vista I would suspect that your set-up is relatively new - my first thought being that perhaps your pc was getting a bit clogged up with junk, which if it is fairly new, should not be the case.
In any case, I would still advise a clean-up, you could run Ccleaner click here this would still clear up some and help things to run quicker.
More RAM, especially with Vista, often helps too.
I am not an expert with Vista, nor anything else for that matter, but this is perhaps to give you a start - this will also attract other members to your plight.