I have had this problem with my keyboard for 2+ years that drives me nuts!!! Let me explain...
Randomly one of the Shift, or Ctrl or Alt buttons (or a combination) apparently gets "left on" and the result is as I type CHARACTERS GET SHOWN IN UPPERCASE. Another effect when trying to select a folder in explorer many folders get selected with subsequent clicks (Ctrl has been left on) or any problems that you may imagine from this issue.
This was happening in my home pc when I had WinXP. I formatted few weeks ago to Win7 x64 - same. Before you ask, I have changed at least 2 motherboards and 3 keyboards on the home pc during the XP time.
It was also happening in BOTH of my previous work's laptops (Dell 620 + 630) and quite often!
From a ton of forum threads (one is 85 pages long) these are the most interesting facts:
1) Many people complain about laptops but the issue is not hardware specific, it's a general one.
@) In laptops, apparently the issue is prevalent when laptop is used with a docking station - true in my case.
3) In one thread, someone said that a major supplier of keyboard parts has f***ed up and send defective parts to hardly every kayboard manufacturer - hard to believe.
4) MS has screwed up the accessibility keys (sticky keys etc) so they essentially malfunction - not hard to believe, explains that the issue is general, not hardware or otherwise specific. Also I've done lots of testing to *really* disable this feature; nothing.
5) The issue stops when Scroll Lock is left on.
The last one is quite interesting, as it seems to really work... This does help a bit, however try to work in excel with the Scroll lock on...
As I said, there are tons of forum threads out there about this but none gives a definite answer or fix, other than the scroll lock approach. That's why I'm asking THW forums for an answer...
After more than 2 years I found the cause: Microsoft's Intellipoint Software
In Intellipoint, you can map a keystroke in a specific mouse button. You can also do this for specific programs only, eg in IE, mouse button 3 can be Ctrl-W (=close tab). Which was my case.
To cut the story short, if you have mapped a Ctrl-something combination on a mouse button and press this button, ctrl stays "on". The same happens with Alt, Shift or a combination of the 3. It was a difficult issue to isolate/reproduce because I had mapped (also previously in XP) Ctrl-W *only* in IE, so after closing tabs in IE, Ctrl was left pressed.
The bug exists in Intellipoint for XP and Win7 (and without testing I bet it happens on Vista) and of course it affects all users who have bought Microsoft Hardware (mice, trackballs) that use intellipoint driver / software...
Emailed Microsoft - let's see what their response is....