clamav scans for windows viruses (only)
Using clam on desktop os x is pretty odd, but if it makes you feel safe...
ClamAV contains definitions for Windows, Linux, and OS X malware.
A search for osx and boonana in it's database shows the following:
Trojan.OSX.Cowhand
OSX.RSPlug-2
Trojan.OSX.OpinionSpy.B
Trojan.OSX.OpinionSpy.A
OSX.RSPlug
Trojan.OSX.iservices.A
Trojan.OSX.iservices.B
OSX.DNSChanger.dmg
OSX.DNSChanger.dmg-1
Trojan.OSX.RSPlug.F.dmg
Trojan.OSX.RSPlug.F.dmg-1
Trojan.OSX.RSPlug.F.dmg-2
Trojan.OSX.RSPlug.F.dmg-3
Trojan.OSX.RSPlug.F.dmg-4
Trojan.OSX.RSPlug.F.dmg-5
Trojan.OSX.RSPlug.G.dmg
Trojan.OSX.RSPlug.G
Exploit.OSX.Safari
OSX.DNSChanger
OSX.Trojan-2
Trojan.OSX.Opener
Trojan.OSX.RSPlug.C
Trojan.OSX.RSPlug.D
OSX.Tored
Trojan.Java.Boonana
Trojan.Java.Boonana-1
Trojan.Java.Boonana-2
Trojan.Java.Boonana-3
Trojan.Java.Boonana-4
Trojan.Java.Boonana-5
Some of these are no longer relevant. The relevant entries represent the variants of the 4 known active Trojans.
I will remove ClamXav when it when it runs out of copies of the dodgy emails from my Time Machine backups to quarantine. The system won't allow me to remove them by hand and I know they were dodgy as I've already described above. However, I'm still concerned that when I email people with PCs I could be sending them malware if I have no AV.
Phishing emails can not infect your system so there is no need to remove the dodgy emails from your Time Machine backups. You are safe to remove ClamXav if you no longer want to use it.
You can manually delete the emails from Time Machine by navigating to the item in Time Machine, secondary clicking the item, and choosing Delete all backups of "item name." You will also have to delete them from ~/Library/Mail and the email server (depending on your settings -> IMAP) to prevent them from returning to your backups.
In ClamXav, you can use the Sentry feature to keep your email clean by setting it to watch ~/Library/Mail and ~/Library/Mail Downloads. It is not recommended to use the quarantine feature when scanning emails ->
http://clamxav.com/index.php?page=prefs
Set the Sentry to scan ~/Downloads to check apps you download if you are unsure. Most of the relevant OS X malware definitions in ClamXav are also included in XProtect built in to Snow Leopard. As suggested by GGJstudios, only download software from known safe sources if you want to avoid the need for AV software. User knowledge is a better defence against trojans than AV software.
If you keep AV software on your system only for on demand scans, ClamXav only uses system resources when you are performing a scan (if you do not use the Sentry). ClamXav is the only AV software without on access scanning so that it will only use resources when running an on demand scan if you wish to do on demand scans from time to time.