utf8 to iso-8859-1

What you've got there is MIME encoded-word syntax using the Q-encoding, which is a variation of the quoted-printable encoding.

Broadly speaking, printable ASCII characters are printed normally, a space is represented by an underscore and everything else is represented in the form '= XX' where XX is the hexadecimal representation of the character number in whatever encoding is specified.

So, using iso-8859-1 (code page 28591), I think the normal text equivalent of your example string would be: Validação da 

See these links for further details:

http://en.wikipedia.org/wiki/MIME

http://en.wikipedia.org/wiki/Quoted-printable

http://www.faqs.org/rfcs/rfc2047.html

If you have a lot of these strings to translate, then there appear to be commercial decoders available but I don't think there's anything in the .NET framework and I couldn't find any free C# utilities (I translated the above string by hand from the table below). However, it might not be too difficult to write a decoder using a regular expression if you can fully understand the specification.

http://en.wikipedia.org/wiki/ISO_8859-1