Anti-Virus programs for linux
There are a few Anti-Virus programs for Linux, as previously stated, and yes, they are mostly there to scan files before they go to a Windoze system.
This can be via e-mail or downloads.
Here are a few examples (as per Google and some from personal experience), I have added some notes if I have used it before.
ClamAV
http://www.clamav.net/
Works quite well, especially when used for a mail system, and is free
Panda
http://www.pandasoftware.com/download/linux/linux.asp
Havent used it.
AVG
http://free.avg.com/ww-en/download?prd=afl
Have used it on Windows. Not the worst AV out there, but haven't used it on Linux.
Sophos
http://www.sophos.com/
It is a commercial system, but you can get a free 3 month demo.
I used this system for a few years to scan all the incomming mail into our corporate system, Never had a virus slip through, just the occasional .ZIP / Password file, but my users knew better than to open them up.
And scary enough this works on Gentoo, so
Avast
http://www.avast.com/eng/download-avast-for-linux-edition.html
The windows version id good, don't know the Linux version
Kaspersky
http://www.kaspersky.com/anti-virus_linux_workstation
I currently use this AV at work, the corporate version, and also the Anti spam/Virus server (Linux) to scan the mail. The Anti-virus side is good, but the Anti-Spam side is a bit lacking.
Out of these I like Sophos, but it is more geared up for corporate users, and is not cheap.
The best thing is play around and see what works best for your system. As far as I know most of the Linux AV's are not "resident" i.e. you will have to tell it to scan a specific file/directory (well I purely only use the command line, as a server must not have X running,,,, EVER). So a bit of scripting will go a long way.
You can even try and combine the scanners, especially the free ones as 2 is better than one, and they shouldn't interfere with each other (std. disclamer applies.... I take no responsibility if you break your system).
I hope this helps.