You want to contact Sigma and find out if they'll still rechip the lens.

Given the age of the lens and the fact that you're not the original owner, they may not be able to rechip it, or might charge for it, but it's worth a shot--the worst they can do is say no. Third party lens makers typically reverse engineer the electronic communication between the lens and the body--they're not privy to the full design the way the OEM is--and chances are that when Canon added the digital EF-S stuff to the mount communication, they broke compatibility with the then-current Sigma lenses.

Some third party lenses are only compatible with the mount system at the time they're made, but may not be so great with the mount as it was ten years ago or will be ten years into the future. While Canon will probably try to make sure their own lenses have backwards/forwards compatibility, they're not going to care about third parties. This is why some third party brands build in the capability for firmware upgrades on their lenses/flashes. But backwards compatibility with older Sigma lenses, particularly on AF function, is known to be an issue. Sigma at the time of the switchover problem, offered to rechip lenses for free, but may no longer do so.