SN.exe is a Strong Name tool that can sign assemblies, manage strong name keys, and generate and verify signatures.  You will typically find it here:

C:Program Files (x86)Microsoft Visual Studio 8SDKv2.0Bin

If you cannot find it there, check “C:Program Files” on 32-bit systems.  Also check the folders corresponding to other versions of Microsoft Visual Studio, such as “Microsoft Visual Studio 9.0” etc.  If you still cannot find it, run a search on your C: drive. 

If SN.exe is not installed on your hard drive, you can download it here:

.NET Framework 2.0 Software Development Kit (SDK)
x86   x64

According to Microsoft, since .NET Framework versions 3.0 and 3.5 are built incrementally on the .NET Framework version 2.0, many of the tools included in the .NET Framework 2.0 SDK are the latest versions available.  But just in case, you can download the newest version of .NET:

.NET Framework 4 redistributable package