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 Download.com page.

Get the updated Firefox at this page.