Impossible to scan 1 file in explorer (W.XP)

Hello,

I just bought Avast-2014 for 3 PCs. (1 and 2 W.XP W.7)

On Windows XP PC, impossible to analyze one single file from Explorer, Avast launches one quick scan of all hard disks.
No problem with W.7.

I did not find anything in the settings.

Thank you for your answers.