If Firefox gives you this error:

"Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system."

First, try the solution found here:

http://www.hintsandtips.com/ShowPost/477/hat.aspx

Then, if Firefox still doesn't work, try this:

Right-click on "My Computer", then click "Properties" > "Advanced" (tab) > "Environment Variables".  Under "System variables", delete the "MOZ_NO_REMOTE" variable.  Click "OK".