Sajjadaziz
Here is 4 more ideas:
I'd have a look at the Mouse Setting in the Windows Control Panel
Have you installed any Add-ins, Try disabling or uninstalling and try again,
Restart Excel to see if that changes the scroll speed
You could also have a look in Excel at Option, Advanced,
Scroll down to display, Enable or Disable Hardware Graphics Acceleration
Try both settings, That may require a restart of Excel to kick in
If that doesn't work, Try starting Excel in Safe Mode
Refer: http://support.microsoft.com/kb/280504
Does the problem persist in Safe Mode?
Try again in Normal Mode
Let us know how you go