Normally, there are two choices on the bottom right hand corner of the antivirus alert window (i.e. Clean and Ignore). However, these two options are not seen. The alert window therefore cannot be closed unless I shut the CIS down.
What version are you using? Is this a clean or an upgrade installation? What language are you using in the interface? When you are not using English try setting it to English and see if the options occur or not.