MBpro suddenly running slow.

I have had my MBPro for around seven months now and have used it for nothing but checking my email and going to around six or seven sites.
( Boring use of a Mac right ? )
I'm puzzled by the sudden slowness that it has though. Tasks such as opening a ( small ) picture in Preview, or playing a song in the background while online results in a spinning pinwheel icon for around 7-8 seconds before anything happens.
From what i understand, Macs don't have to be defragged like PCs, but surely there is some sort of general housekeeping that needs to be done.

I notice that after I cleared my cache, the computer began lagging even worse.

What's going on ?

15" MacBook Pro 2.4 Ghz I-5 4 GB with the latest updates.

Any chance you have used a program called Onyx to run the standard maintenance

Apple - Downloads - System/Disk Utilities - OnyX

I would check activity monitor see which process may be hogging the machine. Repair permissions, check login items under account user. Exit antivirus if any, those tend to run in the background once in a while and takes up resources.

I am not running antivirus. When I bought this thing, I was told that Macs don't get viruses.

I'll check out Onyx. Thanks.

I haven't done anything out of the ordinary to make this thing slow down. Everything is pretty much the way I had it on day one.

BTW I just tried a simple brush stroke in Painter. The lag is at least 2 seconds behind the stroke.

Did you open your Activity Monitor to see what's going on? (as was suggested in a previous reply) Applications -> Utilities -> Activity Monitor.app

Look for any sign of heavy CPU usage. Post back and let us know.

I'm sorry. I have no idea what I'm looking for.
I cleaned the system with Onyx.
Everything checks out O.K.
I looked in the Activity Monitor, but all looks normal ( but the system is doing O.K. currently. )
I'll post back if I have anything happen again.

Thanks