You may also have some scheduled tasks running in the background that you don't need. Also, running several scans everyday, or even every other day, can have a detrimental effect on the life of your CPU, unfortunately I had to learn this the hard way, but I am a better Geeek for it now.
If your processor is a Celeron, or anything similar to it, then this might be somewhat normal symptoms, especially if you perform tasks like DVD encoding, Image Editing, Audio manipulation, and so on. These all hog your resources, and even adding 1Gig of RAM doesn't help out with a smaller processor.
Do some research on the programs that start when you boot up, this may help dramatically, and also, turn the un-needed services to manual instead of automatic, if you are not using them all the time, then they don't need to be started up immediately.
Download this program for a user friendly peek into your starting programs: Startup Control Panel
by, Mike Lin
To view and manage your services, Press- Windows Key+R
-to bring up the Run Box, type in - services.msc
-and the services applet pops up, listing in detail, what is running and what is not.
Open up a browser window, and navigate to this site- Process Library
-anything that you don't recognize, or don't understand, can be searched at that site, and if you don't find it there, use Google like in this example- intitle:aspnet_state.exe
-(that example is in the Proc Lib. by the way)
Hope this helps,