FYI, someone sending mail in your name doesn't mean that any account
of yours has been hacked. Due to the insecure email protocols,
spammers can claim their mail is from whomever the please, including
names that don't even exist. It likely *does* mean that someone you
know has had their computer hacked. The spammers scrape email
addresses out of the victim's address book.
This is a good free anti-malware scanner (it's what CMU recommends):
If you run this on your machine it may be able to detect and remove
malicious software such as viruses, trojans, worms, etc.
You should also have some sort of anti-virus software installed,
such as McAfee, Symantec, etc. Here's a PC magazine article that
reviews some free options:
PCmagazine article on Free anti-malware
The recommend something called Ad-AwareFREE Internet Security 9.0, which also includes a scan and
remove capability similar to MalwareBytes.
By the way, if you think you might have a problem with
malware on your machine, the best fix is to reinstall windows and
make sure that you stay up to date with windows updates, browser
updates (firefox, internet explorer, etc.) Also important nowadays
are patches to Adobe flash and Acrobat reader. You can check if you
are up to date by: