To remove NSIS Media Extensions, (start in Safe Mode http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true
) remove anything Mozilla made (Firefox, Thunderbird, Bon Echo, and the Mozilla Betas), including folders in the Program Files. Navigate to (C:/Program Files/Common Files), and then delete the NSIS folder.
Go into the registry (Start>Run>regedit.exe) and delete all traces of the Mozilla apps (HKEY_LOCAL_MACHINE\SOFTWARE\MOZILLA), (HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla Thunderbird), (HKEY_LOCAL_MACHINE\SOFTWARE\mozilla.org); those are the main targets for the Mozilla orograms. REMOVE ALL OF THEM!
Then in the (HKEY_LOCAL_MACHINE\SOFTWARE\NSIS\MEDIA) registry, delete EVERYTHING in the folder then add a String (Edit> New> String Value) then make the name "OptOut" and the value at "1".
Empty your recycle bin.
Then reboot. Get Firefox from this
Get the updated Firefox at this