Actually changing your password will usually solve this problem if you change it to a longer harder to guess password. We are seeing a lot of this with yahoo. Apparently it is very easy to use a password guesser program on yahoo so any password with fewer than 7 letters is easily guessed. They have now started offering a two stage logon which might help.
I went through several systems which had yahoo mail spam (including my wife's) and couldn't find any malware on the system. It was only after we changed the password to have 8 letters that the spam stopped.
You can try some free online scans if you like but I doubt they will find anything.
Use IE and go to http://eset.com/onlinescan
and click on ESET online Scanner. Accept the terms then press Start (If you get a warning from your browser tell it you want to run it).
# Check Scan Archives
# Push the Start button.
# ESET will then download updates for itself, install itself, and begin scanning your computer. Please be patient as this can take some time.
# When the scan completes, push LIST OF THREATS FOUND
# Push EXPORT TO TEXT FILE , and save the file to your desktop using a unique name, such as ESETScan. Include the contents of this report in your next reply.
# Push the BACK button.
# Push Finish
# Once the scan is completed, you may close the window.
# Use Notepad to open the logfile located at C:\Program Files\EsetOnlineScanner\log.txt
# Copy and paste that log as a reply.
Let's also try the bitdefender quickscan.
When it finishes there is a View Report option at the bottom. Click on it and copy and paste the report (even if it says nothing found).