Code 52 for unrecognizable digital signature for Apple USB iPhone3GS

Had my iPhone 3GS going on 2 years and have just encountered a problem when trying to sync it it iTunes. Windows 7 won't recognize the digital signature. Get this error :

"Windows cannot verify the digital signature for the drivers required for this device. A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source. (Code 52)".

I have uninstalled and reinstalled all iTunes, Apple, QuickTime and Bonjour programs several times, and am getting nowhere! HELP me PLEASE!