Left-click mouse button not working

My mouse has suddenly lost most of its functionality. Most especially, the left button has stopped working altogether except that it occasionally allows me to hightlight text and grab a dialog box to move it. Trying to navigate using the tab and arrow keys and the right mouse button and scroll wheel just aren't cutting it, especially since I need to be able to work in my various graphics programs. I tried a different mouse and changed USB ports, same problem. Restoring to an earlier restore point didn't work, either. The Device Manager says the mouse is working properly. A search for an updated mouse driver says that the best driver for my mouse is already installed. I tried to do a repair install using my CD, but when rebooting, the computer just ignores the CD, and I don't know how to do this manually. I'm on Vista Home Premium, 32-bit. Computer is a Dell Dimension C521. I don't know if it's relevant, but this happened just after I made a download from this page: http://homepage2.nifty.com/~mr_trout/models/poserModel.html However, in CG circles, this is known as a reputable site, and I've downloaded from it before without any problems. Sure would appreciate any help - this is driving me crazy! Thanks.