Hello
I turned DEP off by reading the vistax64 forum.
when i rebooted, a big problem raised.
windows vista ( home premium ) crashed to continue.
i selected safe mode and other F8 choices...but still problem is there.
then I used Vista DVD to repair startup...problem not solved.
I used system restore...but not worked.
DELL company gived a choice to me...it sets all required windows vista files and folders on my another drive...how can i boot it ? 
if i do it, so i can keep avoid my files and downloaded programs ( .exe ) from failed windows ( c:\ ) . 
your forum website is best 
thank you Arsham Farahani