It is of course possible that you have a trojan or spyware, maybe even a virus - it's just rather unlikely. A good start would to be running a bunch of the scripts/utilities found in Onyx - http://www.titanium.free.fr/pg.....opard.html (assuming you're using Leopard, if you use Tiger http://www.titanium.free.fr/pg.....tiger.html is the one you want).
In the Automation tab you'll want to make sure to Repair Permissions, Execute Maintenance Scripts, and clear every possible cache listed.
On the Verify tab, make sure to check the Structure of the Volume, and if it says you need to repair it - do it. In the Cleaning tab, sticking w/ the defaults is safe.
After you've run those utilities and/or scripts in Onyx, reboot. And if you happen to have a desktop full of files - move them elsewhere (the more desktop clutter, the more memory is used).
If none of that solves the problem, consider checking for viruses, spyware etc.