Today a malware on my Mac turned up when I opened my Hotmail, but fortunately I didn't fall for it - it said something like "Apple Security Alert", and then gave a notice that my computer has been infected with viruses, and it showed a bunch of files/programs that have been infected and the number of viruses it supposedly found. If you get something similar don't click on anything, including the Cancel button that it displays.
Do a Force Quite on your browser and restart your computer; check to see if any files were downloaded and Trash them if there were any then empty your Trash. Empty the cache of your browser (I also have a program called MainMenu and I used that to empty my User Cache). Go to System Preferences, then Accounts, and then Login Items and check to see if anything was added to your start up (since I didn't activate the program nothing was added). Do another restart. This seems to have worked for me.