I have heard that Microsoft has really messed up it's recent Keyboard and Mouse Drivers... and such "occasion", which you described, has happened to a number of users. I personaly use older Driver which I would recommend to you as well.
1) Uninstall current driver
2) Find previous driver version (4.x) on the net
3) Download and Install
It should work fine.
If you can't find the other driver, just say and I will send you mine (v4.2). Seriously, there isn't much difference between those two drivers.
BTW, which mouse you are using? Is it one of those new 2-way scrolling type?