I'm not sure what's happening

We have 2 PCs at home sharing a wireless network. On the Desktop PC, I occasionally see a small pop-up over the System Tray indicating something is using the POP3 service. When I notice it, it appears for such a short time I don't have time to record the details. Outlook Express is not active at the time so it's not something I am (intentionally) initiating.

I'm running WinXP SP2 and had a look at Event Viewer with Control Panel Administatrive Tools. I see a record of 'Logon/Logoff' with User 'ANONYMOUS LOGON' and Event ID '540'. There is no-one using the 2nd PC in the house when this is happening.

I use AVG for virus protection and it's all up-to-date. I ran Skybot S&D and it didn't find anything.

Any suggestions where I could look to find out more about this ?

Doing virus/malware scans on a PC that may already be infected may not work. Maybe try scanning the hard disk in a different PC or booting from something like UBCD/UBCD4WIN or a GNU/Linux live CD and doing a scan when the main hard disk is "passive" and not the live system/boot volume. Might also be worth scanning for root kits just in case...

[broken link removed]

Do you know what services you are running?

Is your wireless network protected with adequate encryption (i.e. WPA)? Does this happen around the same time very day or at various times? Have you installed any "free" software recently?