Hi Ali,

>How can I make a digital signature in my software to get rid of this error?

This technology is called Microsoft Authenticode. To emply this yourself you need to get a certificate from a CA that is trusted by your users. Then you need to sign your software with this certificate. The most popular company that issues such certificates is VeriSign:

http://www.verisign.com/products-services/security-services/code-signing/digital-ids-code-signing/index.html

There are companies that are cheaper than VeriSign. However, Authenticode still remains expensive.