Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
When this happens, expand the Summary, Current Status, Show Details, this will give more detailed information on which area is unsecured ?
See image of expanded section, although mine is Secured.
Check the XP task manager what avast Processes are running, avastSvc.exe and avastUI.exe should be running ?