Yesterday i went through a scenario of webenrollment and got to know that When you visit  the certificate enorollment Website ,the client computer downloads and installs the correct version of Xenroll if the correct version of Xenroll is not installed (this was used till windows server 2003) but now with Windows Vista doesnot uses Xenroll. instaed ,Windows Vista uses a set of dual interface Component Object Model(COM) This set of COM object is known as CertEnroll.Xenroll is disablled in Windows Vista .Thereor is you try to manually install Xenroll the installation is unsuccessful