You can check out one of my past posts on how to find out which service pack is currently installed on your SQL Server 2005 instance and on how to get the latest service pack.
However, if you are still experiencing the same problem, even in the case you have installed the latest service pack, then you can try running the sql script called "sysdbupg.sql" which can be found in the SQL Server installation directory under the subfolder "install".
This script upgrades the system database stored procedures from the RTM level. Databases which may be upgraded are:
A big thanks to my good friend Ioannis for bringing this hint to my attention!