Registration problem!

I just bought Avast 5.0
They received my registration file
but when i try installing it, i get an error saying
that the registration file is for an older version of Avast

Can you contact sales (at) avast (dot) com and tell the story?
Maybe they can generate the license again and send it to you.

Did you bought the license on the avast site? Where?
Is your system time and date correct?