Crazy Mouse Problem- Registry or Virus?

In my computer lab, I have 16 computers networked through a hub.The lab computers have Windows 2000 Pro SP2 & Office 2000. They are an local brand.

At random intervals a machine’s mouse pointer will suddenly go crazy. It will open menus & screens, go all over the screen, & take control from the user.

I have: changed mice-including brands, reformatted the hard drive & reinstall everything, installed different software packages, checked for viruses with NAV & housecall.antivirus.com, & changed RAM. I have not noticed any user installed programs on all of the problem machines.

Any & all suggestions welcome.
All new suggestions will be awarded points.