If you are having a wider problem with digital signing, it would be a good idea to do a scan for malware as that can sometime cause problems with cryptographic services.
Also run through *Method 4* in this link:
http://support.microsoft.com/kb/822798
It ensures that all the relevant dlls are registered and also if there are error messages that can help. If you are using Vista you will get not found messages on 3 dlls as they are not present in Vista -initpki.dll gpkcsp.dll slbcsp.dll
You should be able to use copy and paste on the individual lines so save typing and for accuracy.
Problems with these dlls are much more common with XP than Vista.