CAPICOM can be used to digitally sign data, inspect, verify and display their digital signature and/or digital certificate, add or remove certificates to or from the certificate stores, and finally, to encrypt or decrypt data.
CAPICOM Version 126.96.36.199, the latest and last version of CAPICOM, is officially supported on Windows Vista. However, Microsoft has announced that CAPICOM is discontinued and is no longer being developed. Microsoft suggests replacing CAPICOM with .NET Framework's X509 Cryptographic Classes and a number of other alternatives.
CAPICOM was not included in Windows 7. The linked Microsoft article goes into detail.