You just use the standard .NET libraries in System.Security.Cryptography for this now. MD5 has already been broken and is considered insecure, so I'd recommend one of the SHA classes for hashing. Also, the System.Convert.ToBase64String (and FromBase64String) are very handy to use with the cryptographic functions.
We're also recommending people take a look at using ASP.NET Security instead of creating their own authentication table.
If you have an existing login table and need to duplicate the logic of the original hash from DevForce Classic, open a support case and we can send you the source code for the MD5 CryptoFns.