Problem with Private Key in X509

Hi,

    I was trying to create X509Certificate2 object from certificates collection that is stored in Personal store when we open MMC -> Certificates. When I just create it with cert file path and password it works but when I try to retrieve it from My store certificates collection I got error like this:

Key set does not exist: cryptographicException...

I found some information that it's problem with permissions but I gave permission for NetworService as app pool for sharepoint but I have still the same problem. Hmm...I don't know where is the problem;/

Please help.