“MOSS 2007 server installation has a log file called WSS_AdminService.log held in C:\Documents and Settings\Default User\Local Settings\Temp. This is just a log file and it is being written at regular intervals increasing its size. Worse still, because it’s in the C:\Documents and Settings\Default User folder it gets duplicated when a new user logs onto the server console. You can safely delete this file.”

Source: Scribblings of a TechnoBuff - MOSS 2007 Maintenance and Trace logs Removal

Also see:
FullMetal Architect - C: Drive filling up with WSS_AdminService.log growing
Corey Roth [MVP] - Running out of disk space? Check WSS_AdminService.log