If I'm not mistaken, the only way to get a password out of a hash is by using something like rainbow tables since hashes use destructive algorithms that cannot be reversed.
Hash algorithms are not used to cipher, they don't use "destructive algorithms", they're called one-way functions.
On second thought, I do remember working on a .NET app that had symmetric decryption functions in it. One was called "Rijndael" which I was able to successfully decrypt
Rijndael (which is popularly known as AES) is indeed a encryption algorithm, symmetric as you say, that's why it's possible to decrypt a message encrypted with it.
As zeroflaw said, .NET supports different encryption algorithms, and there's none used by default.