@sharif - I learnt that lesson the hard way - DO make backups, even if it's to your desktop. Otherwise you'll be spending hours and hours typing and re-typing all those users' names, passwords and aliases...

Our exchange 2003 recently borked and we lost our aliases... had to retype all of these in again. Luckily I made some printouts beforehand.
Smoothwall and SME Server combination works quite well, never had a problem. It might be overkill, but this way the SME server sits in the DMZ (which I don't need to worry about) and the Smoothie feeds both the DMZ and the green network.
Should the SME server be compromised, then it's a quick hour of reformatting and reinstall, and restore from a good backup without having to do the M$ schlepp. Simply more peace for my mind.
Regards
Lib