Code signing for .net application


i am working in .NET.
i want to enable code signing to my .NET windows and web applications…

i have some other queries regarding code signing…

why we are using the code signing?
what kind of application we will use code signing?
is it possible to use the code signing in .net winforms?
whether code signing is only applicable to ASP.NET Application?
if validity was expired for code signing then what will be my application status?is it shows any message for that?

also i want to know if we puchased code signing it is applicable for all operating systems?
need ur suggestions

plz provide some examples for both winforms(windiows applications) and web applications that with steps…