makecert.exe error: Can't create the key of the subject

I hit a snag while trying to create a certificate for WCF work today. I was following directions on another blog, but every time I tried to create the certificate I would get this error:

Can't create the key of the subject ('<some guid>')

Each time I tried, the GUID would change.

I'm not sure why it is happening, but I did find a workaround. The directions I was following were to create the certificate in the "LocalMachine" certificate store using the following command:

makecert.exe -sr LocalMachine -ss MY -a sha1 -n CN=Client -sky exchange -pe

After some trial and error I discovered I was able to create the certificate in the current user store instead using the following command:

makecert.exe -sr CurrentUser -ss MY -a sha1 -n CN=Client -sky exchange -pe

I thought I would post this message to hopefully save others the effort.
Note to the makecert developers: Please provide more useful error messages.

UPDATE:
I discovered the source of the problem! For some reason, I did not have the appropriate permissions to the directory where the certificates are stored. If you are getting the same error that I was getting, try granting your account (or the admin group, etc.) full control permission to the following directory:

C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys

For more information, see Microsoft KB article Q278381