Check the mouse settings in your control pannel. You may need to either do a windows update to install the latest drivers, or you may need to go to the HP website to download that snychtouch or whatever that software is called (just downloaded it last night). I'm guessing you reformatted or disabled the touchpad software, and at one time you could click by double tapping the mouse.
First check your mouse settings in the control pannel - see if you have to enable the 'double tap' feature.
If nothing is there i would do a windows update to see if that finds the driver you need. If that doesn't work go to the HP website and download their touchpad driver.