So, my PC is sitting here unused for 30 minutes or so, and I see my modem lights going crazy like something is downloading or uploading.

I have nothing actively running that I know of.

I used MalwareBytes and did a full scan - and got a clean bill of health.

I used Spybot Search and Destroy and did a full scan - and got another clean bill of health.

What else can I do to figure out what's going on?

[Edit: I failed to mention that yesterday I got one of those "You're computer is infected - use our virus remover" kind of things that keeps popping up over and over. After running MalwareBytes, I thought it was clean. I ran it again last night before I went to bed, and it found another copy in one of the restore points and removed that too. This morning is when I ran it yet again and found nothing."]

Are you using a wifi router? Someone might have hacked through your security and could be using your internet connection remotely.

Nope, no wifi - just a DSL modem. Sorry, I should have included that info.

It's possible your DSL modem is experiencing a buffer overflow. Make sure you port forward to an echo port at the correct gateway. I had the same problem once, because my DHCP was disabled, check that as well.

Also it could be your assembler misinterpreting data stacks, so you end up with the nuxi problem. For that, check if your object-oriented program can handle class exceptions.

If that doesn't work, check in with me again, I'll show you how to telnet your GNU.

"You sir, are talking gibberish."

o Sorry Chewbubs I didn't know it'd bother you, I'll stop accessing your interwebz now.

Oh no Chewbubba, it was the spirit of John Romero talking through me, I swear!

My modem does that from time to time. It's likely due to your computer still sending stuff to whatever software you have, such as time update requests, checking for windoz updates, antivirus updates, etc. Of the programs I have running my tray, PowerISO, µTorrent, Steam, Avast! and mIRC all check for updates periodically. Even when I'm not actively using mIRC or µTorrent (running, but not doing anything), my lights will still go nuts.

tl;dr government spies in your innernets

probably the libs.

When you're attacking that malware did you restart in safe mode and login to the same account? When I've encountered that stuff a mistake I was making was going into the Windows administrator account as opposed to the HP_administrator account the computer logs into by default. The anti-spyware and AV programs would miss stuff because of the change in account. Just a thought.

I logged in the same account in safe-mode.

And I don't have anything running in the systray either. Lean and clean - the way I like it.

I did install ZoneAlarm hoping that would tell me what was going on. I did show a Java QuickStart plug-in for Firefox that was accessing teh webz, but that was when my browser was running.

And it seems to go nuts when the browser (FF3) is running, even though everything on the page is loaded.

But I was more concerned about when it's going crazy and there are no windows open and nothing in the systray but the volume control.

netstat -a