DONT WORRY! its a scam!.... youll be fine. when you where surfing the web you came across a flash movie or java script that pretends to be a virus scanner that scans your computer. it usually says "You have been infected... you have X harmful files... BLAH BLAH" and its usually the same message for that particular fake online scanner. if you then click "scan my computer" it will take you to their download page and start downloading a .exe which is most probably infected.
youll probably notice that the scanner and the harmful files is for windows. youre on a Mac using Mac OS X so you have nothing to worry about!
but if in the future you come across this on the web that makes out its a Mac OS X app thats scanning your computer, dont click scan as this will probably be the first harmful Mac OS X virus.
be smart when you browse and look out for thsee fake scam virus scanners. remember you actually have to download and install a virus scanner... it doesnt just popup while your surfing and tell you that you have infected files.
BTW here is the online virus scanner your probably talking about ->
http://scanner.defender-scanner.com/3/?adv=2419
DO NOT OPEN THIS LINK ON A PC RUNNING WINDOWS!
a popup appears stating that your computer has slowed down and that you should scan. when you click OK it performs a fake scan. youll notice that its scanning .dll files which are windows files. when its finished scanning the screen dims and it asks you if you want to remove the files in a dialogue box which has a windows UI. anywhere you click on the window will then download Install2419.exe which is most probably full of malware.... AH i love playing with these things on a mac!