Full backup failing, but partial backup successful

We're running server 2003 service pack 2, using symantec backup exec for backups. Every friday we do a full backup and the rest of the days we do a partial, the full backup fails every week.

Writer Name: Active Directory, Writer ID: {B2014C9E-8711-4C5C-A5A9-3CF384484757},
 Last error: The VSS Writer timed out (0x800423f2), State: Failed during thaw operation (10).
Writer Name: Windows Internet Name Service, Writer ID: {F08C1483-8407-4A26-8C26-6C267A629741}, 
Last error: The VSS Writer timed out (0x800423f2), State: Failed during thaw operation (10).

this is the error when the full backup fails, ive checked vss writers using vssadmin list writers commands and both writer are working with no errors. 
Do you know where in the registry these 2 writers would be so I can make the timeout period longer?
Any help is appreciated, thank you.
-standard server