Dock hiding and hot corners no longer work

Question

I am not sure exactly when it began, but for the past 2-4 weeks, my hot corners and dock hiding have been very erratic. At seemingly random times, all the sudden my hot corners stop being active, so when I move my pointer to the corner, the designated task whether it be dashboard, Expose, screen saver, etc. as selected in preferences no longer activates. Dock hiding stops working as well. The dock hides, but when I move my cursor over where the dock is parked it doesn't appear, it stays hidden. I can click command alt d and the dock will appear or disappear, but will not appear by just moving the cursor to the side of the screen where the dock is parked. I have not added any new software lately other than Apple OS 10.6.4 and whatever Safari, iTunes etc updates that keep coming at along at pretty regular intervals. Interestingly I have an iMac at work and at home, and they both are misbehaving in the same way. They are similarly populated with software, although not exactly. When I update one, I update the other. Both started misbehaving at about the same time.
I have rebooted- sometimes works, sometimes not. I have trashed the dock and finder plist- works for awhile, but problem always returns. I have reset the PRAM- works for awhile, but problem always returns.
Any thoughts. I like being able to hide the dock to gain that extra bit of screen space, although the command alt d, is an adequate if not ideal work around. But I really like the hot corners, and I do not know of a work around for that problem.

Answer 1

The university requires all computers to have anti-virus software and recently switched to Sophos from Symantec, in July I believe. That was a new software install. Since it just works in the background and I never actually access it, I had forgotten about that. I hope that is not the problem, as I am required to have antivirus software.
And no I have not created a new user to log in. It is erratic enough that I don't know how long would have to be on to see if the problem recreated itself. Sometimes it seems like just minutes after I start the computer up and sometimes hours. Right now it is still working since starting this morning.