On SBS2003 the drive c: runs out of space. After a search I found that folder "c:\windows\system32\LogFiles\W3SVC1" is getting full with large log files. These logfiles are created by ISS and can safely be deleted.
It looks like several customers encountered this only recently and viewing the "W3SVC1" folder for the 1st time showed in the most cases logflies dated from 2007 or even earlier. Only recent logfiles are large, why does this happen. The answer is quite simple. Recently more and more users are using smartphones to connect to the Virual Directories of Exchange, because this gives the oppertunity to read emails while being out of the office.
A solution is to periodically purge the oldest logfiles. Thsi is easily done by creating a scheduled task with following command:
Forfiles.exe /P C:\WINDOWS\system32\LogFiles\W3SVC1 /M *.log /D -30 /C "Cmd.exe /C del @path"