Michel,
We use the obfuscator that comes with Visual Studio ... it's the free Community Edition of Dotfuscator. It can be accessed from the Tools menu of Visual Studio (2003/2005). Read more about obfuscation here:
http://www.gotdotnet.com/team/dotfuscator/~~Bonnie
>I need to know of the best approach to use to prevent my DLL to be decompile. Anyone would know about a good one to use?