So in summary, no*:
Running Windows on a Mac as a virtual machine won't protect your Windows installation from any viruses it would have otherwise gotten unless you take some other additional quarantining provisions.
*If the data your trying to protect is on the Mac (for example, you use e-mail on the Windows machine but keep critical business documents on the Mac partition), however, it is a little safer, but as Carlosdl pointed out, there's still potential that it's a cross-platform virus that will attack both OS's.
If the Windows virtual machine is going to have contact with the outside world (e-mail, internet access, etc.), then it will be prone to virus infections as any physical machine.