Viewstate MAC address failed in asp.net 2.0 and after page return

I have a site in asp.net 2.0 with a default membership provider. Everything was fine today. When I add a custom google search box on my site, now I get the same error Failed to validate the MAC address of the viewstate. If this application is hosted by a web farm or cluster, ensure that the configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. I found several solutions online that if we installed .net 3.5 SP1 then we get errors like this and either uninstall 3.5 or install SP2 from Asp.net 2.0 to fix the issues. ASp.net 3.5 was installed on my computer, so I uninstalled it. But no success. Please advise me a solution to avoid such problem in asp.net 2.0.