The biggest reason why Windows users tell me they switched from Windows to Mac? Relatively no viruses on the Mac!
When using Boot Camp on Mac OS and installing Windows, one of the first applications to install when getting started is virus protection. McAfee is one good option for Windows 7.
It may not be necessary to install virus software on the Mac, as viruses are not usually written to exploit that OS. If you suspect a problem, Symantec Utilities or MacKeeper are good utilities to install.