Restart while holding the shift key for 30 seconds. Then drag macdefender.app to the trash. Look also at:
/Library/Application Support
and at:
yourhomefolder/Library/Application Support
for anything that looks like macdefender stuff.

Whatever is loading with your login can be disabled from loading:
Open System Preferences > Accounts. Select your account. Select the Login Items tab. Select anything about MacDefender. Click the - sign at the bottom left. This step is not important if you deleted all MacDefender items, as it is only a link to one of those items.

To reassure others, there is no OS X virus. This may be a malware app, but it is not a true virus.

EDIT:
To tdoc: Your answer looks like a question. YahooAnswers is not a discussion, so submit your question for others to answer.

View the original article here