Can't remove highlighting when cursor is moved

When I am editing code in VB.Net 2005 and I move my cursor, the code between
where I was and where I now am stays highlighted. Pressing ESC does nothing.
In order to continue, I must close the window and begin again, but as soon
as I select text, the problem occurs again.

I don't have this problem in the VB6 IDE or in Word. I have a second
machine that uses the same mouse and keyboard through a switch and I don't
have the problem in VB.Net on the second machine, so it's not the mouse or
the keyboard.

I need to be using the machine that's having the problem. Has anyone seen
this before? If so, how do I fix it?

Thanks in advance,

Carol