Not only is Mail not working, there is no iCal in my Applications folder. I've tried re-installing Snow Leopard numerous times but each time it either tells me that my disk needs cleaning (which I've done) or that it can't copy the necessary support files.
However, one interesting thing I discovered doing this was that if you have Snow Leopard already installed and start the Mac OS X Installer, once you type in your admin password it starts installing straight away (i.e. from the desktop) without rebooting from the DVD. However, each time it tries to do this I get one of the messages I listed above. I also get one of those messages if I try installing while booted from the DVD.