Don't bother with anti-virus or anti-spyware software. Just do the following (in no particular order or importance), it's all free. You'll be safe enough.
- Switch on the Firewall and enable Stealth Mode
- Disable any sharing services you don't use
- Use WPA or WPA2 encryption on your wireless network
- Use MAC address access control on your wireless network, and hide the SSID
- Enable the Firewall on your router (if you have one), and change the admin password from the default
- Password protect all the user accounts
- Keep all your Mac software (including Firefox and Office, if you use them) up to date
- Implement a sensible backup strategy
- Set your browser(s) to not open files automatically, disable pop-ups (but allow for websites you trust), reject cookies from sites you aren't actually visiting, and enable "Phishing site detection"
- Don't let your browser remember the passwords to any websites that access your money, or could spend your money on your behalf
- Keep an eye on the Apple Community Websites (such as this one) for news of any serious threats. I'm sure the day someone releases a seriously threatening bit of malware for the Mac, the MacRumors will be one of the first to shout about it.
Also, use common sense. The Mac's superior security can't save you from the deposed Nigerian prince who wants to "give" you £4,000,000,000.
And of course, if you must run Windows on your Mac, then the Windows bit of your machine will require all the usual Windows security stuff.
SL