Screen Freezes on Left Click Outside of Right Click Menu

Whenever I right-click on the start button to open up an options menu and
then left click outside the drop-down menu, the screen freezes. This will
happen if I begin by right-clicking anywhere on the desktop. This behavior
does not seem to occur when I right-click inside the window of an
application, only on the desktop or start button. When the screen freezes,
the drop-down menu(s) stay open and I can move my cursor. I can do a
ctrl-alt-del to bring up the task manager, but when I move my cursor onto it
I get an hourglass and can't select anything. There does not seem to be any
problem if I actually select an option from the drop-down menu after a
right-click. Only solution so far is to cycle power.

This is a Sony Vaio computer and the response from Microcenter (where I
purchased it) was to wipe and reload the OS, which has undergone many
upgrades (none of which fixed the problem) and program installations making
this suggestion undesirable. Besides, everything seems to work except for
this problem. My first choice fix would be to identify and replace/repair
only the damaged file(s).

I have not completely ruled out some virus/spyware culprit, but Norton AV
says I'm fine and the only spyware MS Antispyware finds that I can't get rid
of is Altnet (says it does, but it appears on every scan).

Any help would be be greatly appreciated.