This could be as simple as a malfunctioning contact in your mouse, or possibly a driver issue. I would start by cleaning or temporarily replacing your existing mouse, and see if this resolves the issue. If not, update the mouse driver. If all else fails, try running AdAware (http://www.lavasoftusa.com/software/adaware/) a utility that looks for spyware and other semi-malicious forms of software that aren't usually caught by anti-virus software.
This was first published in March 2004