Attention Star Ranger

[jpub]

The top 3 freebie virus tools out there ar MSE, Avast, and AVG. I'd personally recommend one of the first two, especially Avast, version 5 is shiny.

[bmull]

Along with the programs already listed I've found that malwarebytes is another one to try.  Some infections will not allow it to be installed until you rename the install file.

[Bob Schroeck]

I hate to be the bearer of bad tidings once more, but...  yeah, again.
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.

[paladindythe]

If you suspect your computer has been compromised (by malware), here's a through way to clean it:
The next step is a doozy, backup and reinstall your system.
You'll need an external place to backup your stuff, that's removable storage.
Then wipe your hard disk.  Quick and dirty is a full format (which can take a few hours for a 1TB hard drive, I've found out)
If you're sufficiently paranoid go grab the Ultimate boot CD, and run dban (Derik's Boot and Nuke), it'll take somewhat longer for the basic wipe (which is all you need, since the more advanced techniques are for thwarting Clean Room level data recovery), but you can be sure that nothing's left on the hard disk.
Then start reinstalling your computer, ensuring that the computer is behind a firewall (a home router is preferable, but Windows Firewall--as long as it's turned on--will keep worms out, update Windows fully, probably rebooting a few times in the process, install your anti-malware software of choice, making sure it's updated as well.
Then, do a full scan on your removable media you backed your stuff on, to sanitize it, and then proceed with reinstalling stuff.
Yup, you'll take up an afternoon, after the wiping is done, to get up and running again, but you'll have a clean system.
If you're satisfied that your computer isn't causing the spam, then a) someone has usurped your logon credentials, or you have some 'feature' active with your Yahoo account that you don't use, but a bad guy is, that allows mails to be sent out (or forwarded).
Be through in resetting your credentials, including any knowledge questions to recover your password.
Also, if you have more than one computer in your home network (if you have one), one of them could be infected, and is sniffing your traffic, and pulling logon credentials.  On that thought, how do you access your mail normally?
Looking over all of this, I might seems a bit paranoid--and most of the time you'd be right.  Most of the time.

[ordnance11]

time for the nuclear option
__________________
Into terror!,  Into valour!
Charge ahead! No! Never turn
Yes, it's into the fire we fly
And the devil will burn!
- Scarlett Pimpernell

[Foxboy]

"Nuke it from orbit, it's the only way to be sure."
''We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat
them unconscious and rifle their pockets for new vocabulary.''

-- James Nicoll

[robkelk]

I have to agree - passive and semi-active measures aren't working here.

You wanted to buy a new computer with a new OS anyway, right?
--
Rob Kelk
"Governments have no right to question the loyalty of those who oppose
them. Adversaries remain citizens of the same state, common subjects of
the same sovereign, servants of the same law."

- Michael Ignatieff, addressing Stanford University in 2012