Relay for spam?

[Bill Stanley]

I just got a notification from my ISP that I am sending spam.  I am 
somewhat doubtful but I would like to check my computer anyways.  In the 
last week and a half, I have used Linux exclusively but the only thing 
that I have doubts about is the spam email on this list entitled "An 
Invitation ....", which  I didn't open up.

Now my ISP is complaining about being a relaay for spam.  their email is 
as follows...

**************************************************
Your email account was used to send a huge amount of spam messages 
during this week.  Most likely your computer had been infected by a 
recent virus and now contains a hidden proxy server.
*************************************************

Where should I look for hidden email proxies?  They go on about an 
attachment on what to do but there is no attachment and if there was one 
would probably only contain information relevant to Windows systems.   I 
am suspicious about that email as well.

Bill Stanley