Before I get to my actual issue, let me say this:
I have been running Windows 7 Enterprise 64-bit for some time, and a couple of days ago I decided to "upgrade" to Windows 7 Ultimate 64-bit so that I wouldn't have to do the annoying KMS server bypass thing. I had a spare Win7 Ultimate key anyway, so it wasn't a big deal.
Long story short, the upgrade finished without any issue.
Yesterday, I decided to play Crysis 1 in anticipation of the release of Crysis 3 this coming week. From the get-go, I noticed a strange issue: my crosshair would jump erratically whenever I would press the right mouse button. If I use a different key to aim down the sights, the issue goes away. Interestingly, even when the RMB is not bound to any function, it still causes the erratic movement.
I have a Logitech G5, if that helps.
So far, I have tried:
Uninstalling and re-installing Logitech SetPoint
using a totally different generic Dell mouse
uninstalling SetPoint and all mouse drivers and running the mouse using only the default Windows drivers.
The issue is present in Crysis 2 and Far Cry 3 as well, but interestingly the issue is not present in Portal. I haven't had time to test any other games.
Tomorrow, I'll try other games, as well as deleting the drivers and using the Dell mouse.
This issue wasn't present before the Ultimate upgrade.......