Report Server: The report server cannot decrypt the symmetric key

If you ever come across this error message in SQL Server 2000 Report Server, it is usually caused by one of two things. Either you have recently changed the account that is used to run the ReportServer service, in which case you should follow this article:

http://support.microsoft.com/kb/842421

or you installed or uninstalled some part of .net 2.0 recently, in which case you might have to run the rsactivate tool. go to the directory:
C:\Program Files\Microsoft SQL Server\MSSQL\Reporting
Services\ReportServer\
or the equivalent program path on your computer and type
rsactivate -r -c RSReportServer.config

if rsactivate is not in your path (which it should be after an install) you will find it in
C:\Program Files\Microsoft SQL Server\80\Tools\Binn

or whatever path is the equivalent on your machine