http://alloraconsulting.com/it-solutions/76-windows-server-2008-low-space

http://serverfault.com/questions/8187/whats-the-best-way-to-move-c-users-to-d-users-under-vista-w7

This is a free tool to help you analyse where your disk space is currently being used:

http://www.mindgems.com/products/Folder-Size/Folder-Size.html

This is a short script that is probably best used as an emergency 'free space now' tool:

@echo off
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"
del c:\windows\system32\certlog\*.log
del c:\windows\system32\certlog\*.chk
del c:\windows\system32\certlof\*.jrs
net start "Active Directory Certificate Services"
rem IIS Log Files
Del C:\inetpub\logs\LogFiles\*.log /f /s

If you are building new systems:

http://support.microsoft.com/kb/949977