Virus & Worm question

[Junkman]

I got the following email this morning:

The message or an attachment did not reach the intended recipient(s).

Subject: warning
From: xxxxxxxxxx@earthlink.net
To: crolando@maddog.net
Date: Mon, 13 Nov 2006 07:47:33 -0700

Reason: virus detected (W32/Netsky.B@mm (exact))
Action: deny

I checked the address by searching my sent message box for that email address, and I couldn't find it. Therefore, I assume that this didn't come from my computer, but another computer that put my email address into the email as the sender. To further complicate things, the picture below show a message that occassionally pops up on my screen. I don't know why or understand how this is happening. I have the latest version of Norton running on my machine and it checks email both going and coming. Is my machine infected and I don't know it, nor can Norton find it? Help!!!!