This was an odd one – and Googling for "can't double click" is pretty difficult. You get a lot of folks telling you to do the things you already checked (double click speed, try swapping buttons and see if right-clicking twice works, etc).

I decided to create another user as a test and I found out that the new user could double click just fine, so that means the problem probably lies in HKEY_CURRENT_USER. It was immediately obvious:

HKEY_CURRENT_USER
Control Panel
Mouse

DoubleClickHeight and DoubleClickWidth were blank. Not 0, but blank. Why were they blank?! The system was heavily infected but I don't see why a virus/spyware object would set this. Maybe they thought it was funny. I sure didn't. I tried everything else before figuring this out.

(I think) this means that every double click is counted as an accidental double click because these settings seem to control how far apart your double clicks can be before it counts as a drag. If you set it really high (and your double click rate is really low) you can drag an icon all the way across the screen and click again and it will cancel the drag and activate the object!

Also, why are these REG_SZ instead of REG_DWORD? Is there some point to it being a string?

After I fixed this problem, I went over to a co-worker's machine and blanked out the values.

The 2.3 version of Wordpress is annoying the crap out of me.