Does it happen when you run certain code? If so post the code here. If it just happens randomly, I would say to first try the detect and repair feature of Office, then if that doesn't work, reinstall Office.

Its strange that it happens on two different PCs. Make sure you run an antivirus program, and get all the security updates for your OS and Office from Microsoft.

Also try uninstalling all addins to see if maybe one of them is causing a problem.