Issue on running Photoshop CS3 Lasso Tool

I have a new PC that runs Windows Vista. I am having a problem using PhotoShop CS3. To be more specific, when pressing the Alt key for 5 seconds, the lasso tool automatically goes up. It’s a waste of time especially when I am busy concentrating on etching. This also happens when pressing the mouse button for the same length of time. How can I fix it?
