I was originally looking for options for reduce the size of c:\windows\winsxs which was 13 GB, but this post recommended against it.
I did two things to get me from 4.5 GB to 14 GB free.
First, I ran "compcln.exe" (which is a built-in utility on SBS 2008) and it cleared up about 1 GB of space.
Then I ran the logs clean up batch file as an administrator:
rem Script to clean up disk space on SBS 2008 servers
rem Downloaded from SBSfaq.com
rem V1.0 - March 28th, 2010
rem Certificate Services Logs
net stop "Active Directory Certificate Services"
net start "Active Directory Certificate Services"
rem IIS Log Files
Del C:\inetpub\logs\LogFiles\*.log /f /s
and it cleared up another 9 GB or so in my scenario.