Hi Lobonca,
if you are using Boxcryptor Classic, then you need to install .NET Framework 3.5: http://www.microsoft.com/en-US/download/details.aspx?id=22
If you are using Boxcryptor 2.0, then you need to install .NET Framework 4.0: http://www.microsoft.com/en-US/download/details.aspx?id=17718
Greetings
Gustav