If you can borrow another mouse to get things working...
Open Synaptic and install gpm.
Then add "inputattach --microsoft /dev/ttyS0 &" to your /etc/rc.d/rc.local and /etc/rc.local and reboot.
Voila!
Otherwise...
Log in to your normal user using Tab and Enter to navigate the log-in screen.
Press Alt+F2 and type the name of your terminal emulator - konsole or xterm or whatever.
Type su to become root and enter your root password.
Type apt-get update && apt-get dist-upgrade && apt-get install gpm vim-common vim-enhanced. When this has finished, type vim /etc/rc.local.
Use the down arrow key to get to the first empty line at the bottom of the file. Press Insert and type inputattach --microsoft /dev/ttyS0 & followed by Enter. Then press Esc and type :w followed by :q.
Repeat the last step with vim /etc/rc.d/rc.local.
Type reboot and press Enter.
See inputattach --help for other options (other mice).
If you want you could make a dedicated initscript called, say, serialMouse, to stop and start the attachment, set its requirements and get chkconfig to put it in the boot order (or even use PCC to turn it on as a service), but rc.local is probably good enough for most people.
