Unable to log after the first scan of Avast Home

[Zen3349]

Hi,
My sister installed Avast Home on her laptop because she just get a web connection and she was looking for safety.
The software request a full scan of the system so she did it.
The software told her that it found virus in several file and proposed to store them in quarantine so she accepted.
After reboot she is getting to the logging window of Windows XP but when she enter her password, the system is looping back to the looging window.
How to avoid this ?
Did Avast warn about "false virus" ? (She never connects to the web before).

Expecting a quick feedback,

Zen3349

[Barr_y]

Hi Zen Try telling her to start in safe mode. Then restore her computer before she downloaded Avast. I am sure when it is back to normal, try downloading Avast again and reinstall. When you get the opportunity, try a boot scan first. Starting in safe mode you may have to keep pressing F8 on start-up.

[Spiritsongs]

   Hi Zen :

     It's hard to know WHERE to start . Before installing Avast,
     when your sister got to the Microsoft "Login" page, did
     she successfully enter her password and the desktop
     screen appeared ? Is she the ONLY one using the laptop ?
     Usually when you get a laptop, there is already an
     antivirus program on it; if this is correct, what antivirus
     was originally on the laptop ? Was it COMPLETELY
     REMOVED PRIOR to installing Avast ? If yes, specifically,
     whay steps did she take removing it ?