Excel 2010 Shortcut Menus Disabled

I am running MS Office 2010 on Windows XP. This morning when I opened an Excel spreadsheet I created last week (very simple one, no VBA code, just a table for me to manually enter info), my right-click shortcut menu was disabled. It seems to be disabled for cells, rows, columns, and sheets. When I found this, I opened a new, blank spreadsheet, and I had no right-click menu there, either. I have not modified any settings in excel, and I have used no VBA code in this spreadsheet, though I did test-run a spreadsheet on Friday for a coworker that uses extensive VBA code which controls the UI. I've read some threads on message boards that seem to indicate that this may be a software problem. Any ideas? By the way - my coworker can run the same spreadsheet that I test-ran and he doesn't have this issue.