Susan,
I'm assuming that the 10 pc's didn't start doing this at the same time. If they are networked, or if you are otherwise sharing files between them, they may have inherited a virus. Viruses can corrupt the executables.
SECOND POSSIBLE SOURCE
The only other thing that I can think of is to check the 10 machines for a single common program (recently installed) that is not installed on the other 50 machines.
WHY THIS CAN BE A PROBLEM
When installing a new program, dll's and vxd's are copied to the system directory. These copied files may overwrite older versions of the system files. Programs written with the older version of the dll may not work with the newer version.
SOLUTION?
I suggest the following:
Scan with the most up-to-date virus definitions available from your anti-virus provider.
If that doesn't solve your problem,obtain the latest version of your mouse driver and the latest version of your video driver.
Load one of the problem machines with the mouse driver.
Load one