Some friends sad that they had problems with e-mails! When thy try to send e-mail(yahoo service) a pop window appear and says: "You are not authorizated to perform that action"
yeah, it may be temporal only.. Are you using that computer as a guest? which email software are you using? Outlook, Apple mail maybe?
If it's a virus then you would have recognize it right away because that way you can't do anything. No opening up the task manager, nothing.
the new version of yahoo uses an external application to upload larger file of 100 mb, it was installed with a request from a user as a normal application , but do not install it . you can delete it and use inbuilt yahoo uploader. you can uninstall that application in yahoo at the bottom left corner.