iMac freezes after login

I've had my iMac for about a year now, and lately after I logon the desktop appears immediately followed by the beachball. It doesn't happen every time I turn it on or even every other time, but still enough to worry me and for long enough that I have to manually shut down. I've repaired disk permissions and ran Sophos. The activity monitor shows the CPU usage to be very low and healthy so I have no idea what's causing the freezes. Any help from someone with more than my super minimal knowledge would be greatly appreciated.

http://mmlweb.rutgers.edu/music127/b...ash_freeze.htm

you may find some help hear. best of luck !