In some cases its been known to happen where Net Extender can’t  completely uninstall due to corruption.
In these cases it is not seen in Add/remove programs but if you try to install you get an error that you need to uninstall the old version.
Here are directions on how to do that.

Directions to uninstall Net Extender on Windows 7 if the install is corrupted:

1. Reboot the PC (to release any locks)
2. Find another 64 bit windows (assuming this is win7 is 64 bit) with Nx installed, copy all files under “C:\Program Files (x86)\SonicWALL\SSL-VPN\NetExtender” to your machine.
3. Execute following commands in sequence:
NEService64 –i uninstdrv
NEService64 –i uninstall
NEService64 –i instdrv
NEService64 –i install
NEService64 –i uninstdrv
NEService64 –i uninstall
4. Remove all files you copied.