All of the suggestions here are for Windows virus checkers. None of these work on Mac OSX. However if you were running Windows on your Mac as you can easily do, then they would be useful for the Windows part.
For Mac OSX there are no viruses, spyware or malware in existence. So apart from MS Word viruses, which do nothing to the Mac, but can be passed on to PC's, there is nothing to check for.
There are two Mac Trojans in existence. These are damaging programmes hidden inside real software. They install because you give the real software permission to install. However both of these come from sites designed to pirate software. So stick with rule 1. Only buy or download software, tunes, films etc. from reputable companies.
If you do want an inexpensive and good checker for the Mac, buy Intego Virus Barrier. (the free trial may convince you)