Você precisa estar executando o avast para abrir a Quarentena (pois os arquivos são encriptados).
Em inglês:
Overinstallation can solve the problem and you won’t lose your programs, settings, data, files, etc.
Just choose ‘Repair’ installation of Windows and install ‘over’ the old installation.
http://support.microsoft.com/default.aspx?scid=kb;EN-US;315341
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q314058
http://support.microsoft.com/?scid=kb%3Ben-us%3B315341&x=15&y=0
http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx