I can't click.

I installed Ubuntu 10.04 on my Dell computer using Wubi, so I am dual-booting with Windows XP. After finally getting into Ubuntu for the first time, the ability to click went away after I opened something. I could still move the mouse, and any button that I click on would be briefly highlighted, as if acknowledging that it had been pressed, but nothing would happen. I could still use the "Enter" key to substitute for clicking, but I kinda like clicking things. I fully updated, but still nothing. So I removed Ubuntu and reinstalled it (twice) only to encounter the exact same problem. I would sometimes spontaneously regain the ability to click for no apparent reason, but it would go away as soon as I moved onto a different window. I happen to have a DPI button built into my mouse, and whenever I press it - thus changing the DPI - I am magically able to click, although it still goes away in just the same way. I've searched the internet relentlessly for the answer, and the only thing I could find was a work-around which I already found out; changing the DPI. That's not really a very good work-around, though. What do I do?

Thanks, nerds.