Being back from a nice vacation in DomRep I just had an interesting issue.
Upgrading to Windows 2000 SP4 from Windows 98 or Windows Me you might see problems when using the Navision Software with the German Elster Services (used to interface with the German tax institutions). Importing the NavisionExample.Pfx (supplied for Testing ONLY!) works fine, but when you try to export the certificate you get "can't find the private key for this certificate" even though CertUI does show the "You have a private key for this certificate". The pfx works fine on newly installed machines and also on all machines running Windows XP and Windows Server 2003 (new install AND upgraded).
Solution: Make sure you install the available updates from Windowsupdate. The update from MS04-011 will provide the needed updates to handle the pfx correctly.