Cursor anchoring randomly

When I am writing and editing text (doesn't seem to matter what package I'm
in - Word, Outlook Express, even this web page) my cursor will sometimes drop
an anchor (like you do when you are about to highlight text to copy or cut
for example) and just will not release the anchor. I've tried doing a dummy
cut and paste (cutting a space and re-pasting it back where it came from) but
to no avail. The other thing I've noticed when this strange phenomenon
occurs is that the pop-up menu that normally appears when I right click the
mouse no longer appears. I was wondering if a critical file to do with
mouse control has been corrupted or
removed by my anti-virus software (AVG free edition). The only way I can
solve the problem is to shut down the computer and re-boot. I've also
noticed tht when this phenomenon occurs the "Restart" option on my computer
shut down/restart menu does not work - it's shut down or nothing. Anyone out
there got any clues?
.