Rsa Algorithm-Help i am going nuts

I need help with this code i have suessfully able to encrypt File and decrypt file using the DesCryptoserviceProvider but everytime i try to use the RSACryptoServiceProvider i keep getting this exception

Server Error in '/Cryptology' Application.
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CryptoAPI cryptographic service provider (CSP) for this implementation could not be acquired.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Security.Cryptography.CryptographicException: CryptoAPI cryptographic service provider (CSP) for this implementation could not be acquired
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This is how i intialised the rsaCryptoServicePorvider

Dim service As New RSACryptoServiceProvider()----exception occurs here------


hope someone out there can help me cos i realy need to use rsa alogorithm to encrypt my files.....

*bump*