Sharepoint 2007 json is undefined error with IE8

Hi all some users with IE8reported a javascript error that displays

json is开发者_StackOverflow undefined

whenever they browse any page in our sharepoint portal.

other users with IE8 did not report that erro and said that everything is OK.

did anyone encounter such an error before ?

thanks

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Thanks djeeg. the thing I found that those users had some plugins like Divx installed to their browsers. when they removed the plugins everything worked fine.