Maybe something running in the background is causing the problem.  Do you
regularly scan with up to date antivirus and spyware programs?  It could
also be that the menu speed is set too fast--see next paragraph.

As a work around, you could install TweakUI and one of its tweaks causes the
menu not to do anything until the user left clicks on something.  For
example, you could hover over Programs but the Programs' menu would not
appear until you left clicked on Programs.  Once Tweak is installed, go to
the Mouse tab, Menu Speed, and set the slowest speed.  Note:  you could also
experiment and see if the Menu Speed is just set too fast.

Tweak UI is available from:
http://www.microsoft.com/ntworkstation/downloads/PowerToys/Networking/NTTweakUI.asp
Right-click the TweakUI .inf file and select install.  Should a notice
appear saying a disk is required, click OK and another screen appears. Click
browse, go to the folder containing Tweak and click OK and OK.