More info required.
What operating system is being used? (Same on all affected PC's???)
What type of mouse? (Same on all affected PC's???)
Are you using Microsoft Intellipoint or Intellimouse software? If so, is the same software (and version) used on all affected PC's?
If it's happening to all computers in your office, are they networked to a common server?
What companion Microsoft software is being used (ie., Office)? (Same on all affected PC's ???)
When did the problem first start and was it the same on all PC's immediately?