More information please. Which OS are you using? Which version of Office? How much RAM do you have? What is the hard drive capacity and how much free space does it have? Which specific Mac are you using? How many apps are open when this happens and which apps are they?
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Jon
Mac Pro Quad 2.66 GHz, one 500 GB Hitachi HD, three 320 GB Hitachi HDs, 5 GB RAM, OS 10.5.7
Epson SP 1280, LaCie 80 GB FW drive, second internal DVD drive (Pioneer), Photoshop CS3, Office 2008,
Nikon LS 8000 scanner
Apple 23" Cinema Display