10 GB OS is small but should be workable if you move some stuff.
A few ideas:
1) You can use MS's Search utility to search for large, or recently modified
files to see where the space is going.
2) Make sure exchange is being backed up and you are not collecting lots of
exchange log files (these are removed when you back exchange up).
3) Use a third party utility like TreeSize to get a report on your disk
space usage that you can track over a few weeks.
4) See the following MS page for help documents and download "Moving Data
Folders for Windows SmallBusiness Server 2003" for other ways of cleaning up
Also do a groups.google.com search for "sbs2003 drive space low" and you'll
get lots of info on this very common issue.