At first, you need to enable serial ports in your BIOS and wire them up to the case front (if they aren't allready).
Make sure you have the following in your /boot/grub/menu.lst:
serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
terminal --timeout=10 serial console
# kopt=root= ro console=tty0 console=ttyS0,115200n8
Check that "ttyS0" is in /etc/securetty, remove /etc/ioctl.save, if its there.
Last change goes to /etc/inittab, add there:
T0:23:respawn:/sbin/getty -L ttyS0 115200 vt100
Now you can use a terminal program like minicom with 115200 baud, 8n1 and no flow control set.
Have fun and much luck on recovering your machine!