I'm having the same problem described in this, unresolved, thread on MS ANSWERS:
I have a MS Wireless Laser Mouse 6000 v2.0. Windows 7 Professional 64-bit.
Most dropdown controls and scrollbars will behave as if the mouse button is being held down.
I should note the keyboard will often act as if the arrow keys are being pressed, as well.
This is only on Windows 7. I use the same mouse on a laptop running XP, with no issues. I've tried another mouse, it behaves the same. So I suspect it isn't a hardware issue.
I've installed and uninstalled the mouse/keyboard software from MS, makes no difference.
Interestingly, when I remote to the system from an XP box using the same mouse/keyboard, the problem doesn't appear.
So I think I've thoroughly narrowed this down to the Windows 7 OS itself not working well with this, or any, mouse, on my system.