best I can suggest -
download the volume activation management tool (http://www.microsoft.com/en-us/download/details.aspx?id=11936)
select 'product keys' option (last one)
enter what you think the product key is, click verify
if it fails to verify then try another option that it might be and hit verify again, if there are only one or two letters you can't read it shouldn't take too long to find the right ones.