Running ESET Nod32 on both processors in Win XP

When I first downloaded Eset Nod32 a couple years ago, it ran on both processors at the same time. It checked the C: disk on one processor while running Nod32 for the other disks on the other processor at the same time, essentially cutting runtime in half by splitting the job over two processors.

After a few months of running it this way, something happened and it would only run it all on one processor.

How can I run it on both processors like before?