Vista Tutorial - key prob

Two days ago I was using my computer and it hang up,nothing strange,reseted
it and oops: what's this message asking to enter my key to activate windows?I
took a look on the case sticky and entered it down but it tells me that its
not an authentic key (god it's a sticky of Microsoft itself ) how this could
be?
Im really disappointed please help I dont have much time:it says 12 days
left !
I have an Intel e4300 based comp with 2 g sys ram a geforce 7950 512mb
Kaspersky Antiv 6 win Vista home premium w: all updates from microsoft.
all worked fine aboout 1.5 month then booom!!!!!!!!!!!!!!!!!