itunes restore error code 21

my iphone4 is blocked since (I suppose) it tries auto-instal the new IOS.

Itunes recognise that it needs to be restored but the process allways ends

with an error code 21.

I tried every thing I could find on forums (security software off, firewall off, latest versions of itunes and

PC Os) ... but nothing solved my problem.

Any new idea is welcomed!