Ok, first of all the online scanners are free and here is a list of popular virus scanners:
http://www.pandasoftware.com/actives..._principal.htm ;
http://www.ravantivirus.com/scan/indexie.php ;
http://housecall.trendmicro.com/ ; and
http://www.bitdefender.com/scan/licence.php
More importantly I would recommend the following as well...a trojan scan (its free as well) at
http://www.trojanscan.com/ and that you get a good spyware program like spybot or spysweeper and scan your PC for pests and tags as well. As far as the account goes, outside of trying to steal it back, consider it gone. Consider it a valuable lesson and move on by creating a new one. I recommend that you use a better password as well. Yahoo allows a maximum of 13 characters in a password and you should use them all - and include a mixture of numbers and letters. This makes it very difficult and exhausting for those trying to "hack" the ID. You may also contact Yahoo to report the offense, but I am not sure what if anything they will do about it since you don't know the culprit's ID. They could however close the account completely, which means that after a period of six moths it will become available to use again ( on a first come, first serve basis).
To contact Yahoo visit
http://add.yahoo.com/fast/help/us/privacy/cgi_feedback
P.S. I recommend visiting
http://security.yahoo.com/ to learn more about internet security.