Do I need to have TWO anti-virus subscriptions on my iMac?

I bought my iMac over a year ago.  It had Norton antivirus for Mac on it.  It expired.  I renewed.

I also have Parallels 4 on it.  McAfee Antivirus is saying I need to renew it.  Do I need to have TWO active anti-virus (one for mac and one for Windows?).  Money is tight and I don't want to pay for a Windows anti-virus if I don't need to.