Viruses are typically transmitted via e-mail, so unless you're e-mailing between the two machines, the chances of a virus moving from one machine to the other are almost nil. Add to that the fact that a PC virus probably won't infect a Mac, and vice versa, and I'd say you're pretty safe.
The same is true for worms, which don't require e-mail... again, a worm that works on a PC isn't going to work on a Mac, and vice versa.
Just make sure that your router's firewall is running and configured properly, and make sure that your service packs are all up-to-date on both the Mac and the PC (I assume it's running Windows XP SP-2), and you should be cool.
Oh, and back up your data on a regular basis.