Did you make partitions before you installed?
If so did you make a primary partition or just a bunch of logical drives in extended. Basically if you made just logicals thats why it wont boot. If your lucky, there may be a really small amount of space at the start of the drive (usually 2 or 3mb), that you could make a primary partition on. Then simply boot up a rescuecd/boot floppy and alter /etc/fstab to reflect the new change to mount locations.
Alternatively you could use partition magic to convert your / (or /boot if you seperated them out) partition to primary, but you'll still need to change fstab to reflect the geometry change.