"CryptographicException: Access is Denied" When Calling RSACryptoServiceProvider.ImportParameters

I am porting an ASP.Net application from 1.1 to 2.0 and am running into an error. The application creates a digital signature using SHA1 and RSA so needs to import the signing key. When calling RSACryptoServiceProvider.ImportParameters I get the following error:
[CryptographicException: Access is denied.]   System.Security.Cryptography.CryptographicException.ThrowCryptogaphicException(Int32 hr) +33   System.Security.Cryptography.Utils._ImportKey(SafeProvHandle hCSP, Int32 keyNumber, CspProviderFlags flags, Object cspObject, SafeKeyHandle& hKey) +0   System.Security.Cryptography.RSACryptoServiceProvider.ImportParameters(RSAParameters parameters) +301
The relevant ...