Hi, and welcome to the forum!
That sounds very odd, and I'm trying to visualize the issue here... If the workbook is maximized on your screen, and it just doesn't seem to be accepting mouse clicks, I guess I'd wonder two things:
- Is the mouse actually registering the clicks to Excel? (Can you test with a different mouse?)
- Is it time to run Detect and Repair on the office installation. To do this, go into Control Panel-->Programs and Features-->Microsoft Office 2007 and click "Change". Choose Repair and let it run.