CAPICOM.dll has been removed from the Microsoft Windows SDK for Windows 7 and .NET Framework 3.5 SP1: BETA and should be considered a deprecated component. You can find information on alternatives to using CAPICOM here - http://msdn.microsoft.com/en-us/library/cc778518(VS.85).aspx. This MSDN article tells how to use the .NET Framework to implement security features for the following:
If you need to use CAPICOM.dll, you can download it separately from http://www.microsoft.com/downloads/details.aspx?FamilyID=860EE43A-A843-462F-ABB5-FF88EA5896F6. This download contains the redistributable files for CAPICOM 220.127.116.11 and samples of its use.