A few tips that may or may not help. Also if it is possible can you remember what you were doing with you laptop around the time you noticed it getting slow.
Make sure that you have emptied your "waste basket", a few years ago a friend of a friend noticed the same problem as you and it turn out, they thought that by dragging items to the "waste basket" it would automatically deleted them. Obviously that is not true.
What ever your browser is make sure that the history has been deleted every once in a while.
Mostly macs should be fine, but if the worst comes to the worst you can also just reinstall the OS using the disks that came with your laptop, JUST MAKE SURE YOU HAVE A BACKUP OF ALL YOUR PERSONAL FILES.
As for Anti virus, generally macs don't need anti virus software, but it doesn't mean that they are invulnerable. ClamAV is one anti virus that a friend of mine pointed me to ages ago. Never used it, but it is one of the few free ones for Mac OS X out there.
Disclaimer
I do not take any responsibility for the actions taken as a result of the tips posted in this message.
Have you ever danced with the Devil in the Pale Moonlight!