why boot auto scan disable of my avast home edition

Avast Free Antivirus / Premium Security
1

1/update 4.7 to 4.8,and boot auto scan disable.
2/I downloaded on avast.com and installed avast home edition 4.8
and registered,enter my key.but,boot auto scan disable too.
please help me.How can I use boot-scan?
thanks and best regards.

2

Do you have Win XP or Vista 64bit? No bootscan in 64bit

3

I see.

Thanks.My leptop’s system is 64bit.