Vista working fine then very slow

Hello,

I bought a laptop a few months ago with Vista. It was working very well
until a week ago. I shut it down one night and starting up the next day took
something like half an hour. The web browser needs a full minute to open
up... every thing is so slow...
I've ran a virus scan, ran a defrag, blocked or deleted useless start-up
programs etc... and it's still slow. I have no idea what happened...

Any ideas and help would be welcomed.

Thanks,

Aurélien