Com issue halts FFXI >_< Help pleaseFollow

Whenever I start up my computer, before windows actually loads up a black screen pops up showing that it has detected an error on my harddrive, click a botton to continue... No problems showed up till yesterday.. when I click on Play on playonline to play XI my screen goes black and I cant do anything... This has also happend when I went to log onto my Internet -Mozilla- and same thing happens... Screen goes black and cant do anything... I have scanned my whole harddrive with windows scanner (in Helpspot check) and done a full com scan with Nortans and nothing shows up... If anyone can give me any advice it would be greatly appreciated.. thanks for your time...