Mike, I had the exact same problem with not being able to select text
in messages in Outlook Express (I just got a new motherboard too)
except with a couple more strange symptoms, one being that none of my
Windows programs would open in the 'active' state, i.e., they would
always open inactive (title bar greyed out). And whenever I visited
Google, a long string of zeros would suddenly fill the search box. In
fact, whenever I opened any program that had some kind of text or
input box, a long string of zeros would fill it up (for example,
clicking on Start >> Run would do the same). All of this totally
freaked me out and I spent half a day doing all kinds of virus checks,
spyware checks, registry checks and cleanups, all to no avail. I
eventually discovered that the problem was the new keyboard I had
gotten along with my new motherboard. The Ins/0 key on the number pad
had somehow become stuck in the down position. I tapped on it to get
it back up, rebooted and all the problems were cleared up.

(*In my best Homer Simpson*)... D'OH!!!