Before Windows XP Service Pack 3 was released, there was no SHA2 functionality within Windows XP. With the release of Service Pack 3 some limited functionality was added to the crypto module rsaenh.dll. Windows Server 2003 Service Pack 2 does not ship with support for SHA2. It took me ages to figure this out, as you can get some strange errors or page not found etc and it will confuse you as to why you cant access a certain website.
On Windows XP ensure that you install Service pack 3 this will give you some limited functionality that was added to the crypto module rsaenh.dll.
On Windows 2003 download http://support.microsoft.com/kb/938397Please be aware that it is not available via Windows Update it needs to be requested via the “View and request hotfix downloads” link. You may also need to install http://support.microsoft.com/kb/968730if you need to enroll from a SHA2 certificate authority and process SHA2 certificates, KB 968730 completely supersedes KB 938397.