You can boot into runlevel 1. You do this by pressing "e" when you see the GRUB screen. Navigate to the kernel section using the arrow keys and press "e" again. At the end of that line, add a space and the number 1. After that press "b" to boot the system into runlevel 1. Once you are in runlevel 1, you can fsck.
An alternative would be to boot from the first installation disc (or the dvd) and get into rescue mode. More information is available here