No bomb threats - just advice - FIREWALL and serious ANTIVIRUS as well as SYBOT and ADAWARE are urgently needed....
Have fun...
Why?
The built-in firewall is quite sophisticated. As an experiment, I have been running my Mac (OS X.4.4 through X.4.10) for over a year with the Firewall turned off! There are attempts... but nothing gets through. With the firewall turned on, I have visited security sites that evaluate the security of visiting computers... they don't even see my Mac.
What would the Mac "Spybot" and "Adaware" look for? Six years in the wild, unprotected, with over 23,000,000 users, and there are still ZERO viruses and ZERO ad-ware and ZERO spy-ware for Mac OS X found in the wild. The available Mac anti-virus ware essentially only looks for Windows viruses.
Unless you willingly install the spyware and the adware on your Mac, it can't be installed.
None of the four or so proof-of-concept "Viruses" that has been produced and seen only in security labs will currently work on Tiger... much less so for the upcoming Leopard. None of them has spread for lack of a viable vector... if you can't automatically install it, the only way to get it on a Mac is to fool the user into installing it for you.
When malware appears in the wild, THEN we will consider what anti-malware steps need to be taken.
This being said, if you run Windows OS on your Mac, you WILL need all of the above... to protect your Windows files.