What operating system and version?
Is it cs5 or cs5.1?
If it's cs5, do you have it updated to 12.04?
it's doing what it's supposed to do only on right click or click and hold.
The behavior is the same for left and right click and hold. Nothing unusual so far. The only oddity is that it misinterprets your clicks and that points to an issue with your mouse driver and/or Windows' own mouse settings. Check that. In particular mice with custom buttons may be prone to such behaviors...
Yes, I would check the mouse settings.
As for "I've tried pressing both sets of ctrl + shift + alt at the same time repeatedly (was told that sometimes fixes bugs)", be aware that you need to do it immediately after you clicked on Photoshop's icon to start it up, and you should get a prompt asking you if you want to delete your preferences.