There are actually viruses for the mac such as OSX/Leap-A (also known as OSX/Oompa-A) which spreads via instant messaging, Some owners of Mac computers have held the belief that Mac OS X is incapable of harboring computer viruses, but this is simply incorrect.

One reason switchers are flocking to OS X is because of the lack of prevalence of malware. Folks are tired of viruses, worms, trojans, etc. hammering their systems. They are even more harrowed by having to maintain vigilance over their anti-virus programs, hoping they are not too far out of sync with the current “DAT”. However, switching to run OS X to avoid running anti-virus programs may not be the wisest choice as it does have exploits which effect it, the biggest issue at this time being rootkits the same as Linux

The rootkit detector I use is a command line tool available at the following URL :
http://www.chkrootkit.org/

Find out more about rootkits @ http://en.wikipedia.org/wiki/Rootkit

The Viral threat on Linux and OSX are not as widespread as on Windows and not a major concern for most due to the manner in which the OS is constructed, they simply are not effected in the same manner, Ive not used antivirus for the last 4 years.
Now while your at it quickly format that Toshiba laptop and try Ubuntu Linux (although you can run it from the CD just for a quick look) another great posix compatible OS which is much faster than Windows and more secure