If you're running Reporting Services 2000 and you accidentally change the password or remove the original account used by Reporting Services, you will have to update this information.
With Reporting Services 2005, there's a nice GUI utility that you can use to do this, but for Reporting Services 2000, you'll need the rsconfig.exe command line utility which can be found, in a typical installation, at: <drive>:\Program Files\Microsoft SQL Server\80\Tools\Binn
To update the account or password information, the following command will probably do:
rsconfig -c -s servername -d ReportServer -a sql -u username -p password -t
Useful to know...odd that with all of the RS development I've done in the past, I've never had to use this utility once until today.