I think it is the bluetooth keyboard. Actually, the bluetooth dongle.
I've had bluetooth keyboards cause all sorts of errant keypresses. I use an Apple Wireless keyboard for my living room HTPC. I knew I could use the keyboard (on both my laptop and my Mac Pro at work) without issue, but on my HTPC it would "type" things. Mostly, it was "sticky" keys (I'd type one i, and it would keep typing "iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii" forever and ever). But I also saw errant keypresses, that didn't seem to be triggered by use of the keyboard.
But, I was using an Apple keyboard on a Windows box, and it was acting flaky, and my house is full of radio interference and... So, there were a ton of things to blame (and I didn't have another PC handy with bluetooth to test it out).
Then, one day, I was getting errant keypresses (I think the start key, in my case) when the keyboard was totally powered off. I even took the batteries out to be sure.
The bluetooth adapter, of course! So, first thing I did was figure out what adapter I had (really, the chipset, not the brand) and started digging. Then, I figured for $19 or whatever, I could just get another one (which would be handy for testing with the other PC anyway). When the new one came, I switched to it anyway, but before it came, I figured out the solution was drivers. I'd installed the "manufacturer provided" drivers (not even the ones on the disc like an animal, but ones from their website). Found a bunch of forum posts and stuff and ended up removing the drivers (which was a big pain to do, if memory serves, because the uninstaller didn't remove everything), and letting Windows do what it wanted with the dongle "fresh", and it worked right from then on.
So... Check your bluetooth doohickey. Unplug it completely, or disable it in the BIOS if it is onboard. Wait a day or a week or whatever. See if it happens. If not, then you know your culprit.