PB Lagging?

For the past two weeks or so, my PB has been lagging. Like nothing has crashed but just the load times have been severely lengthened. I have the activity monitor open at all times, and a few weeks ago my computer has been running low. But recently it's been running high. Even when I have just iTunes and iChat and Safari open. That's it. I don't understand why it's running so high, but the only remedy is to restart. Which is a pain in the arse. Also I thought it was because of having little space left on my harddrive but it wasn't that. because i deleted 20 gigs of videos and such and it still runs high. Its beginning to annoy me, so if you guys have any suggestions. Feel free to express them.

Thanks in advance,
EY