[olug] Security breach?

[Jack]

I'm running MandrivaLinux x64 2011 (KDE4) updated behind a commercial Trendnet 
router.

I notice I've begun receiving spam emails supposedly from people in my address 
book (thunderbird).  When I run 'top' I don't find any obvious intruder 
files.  Having received three of these now, all from different addresses and 
people, I suspect my computer is compromised.

This prompted me to check my security settings.  Turns out I had left the 
firewall down from my last full re-install a couple of weeks ago.  It's back up.

Any suggestions for ridding a Linux system of malware?

     Jack