actually you don't want to be able to decrypt it.
what you do is encrypt the password and save that in the database
when the user logs in, encrypt the password the input on the login screen and compare that to the encrypted password in the database
here is a sample for using MD5 hash to encrypt
public string Encrypt(string inp) {
MD5CryptoServiceProvider hasher = new MD5CryptoServiceProvider();
byte[] tBytes = Encoding.ASCII.GetBytes(inp);
byte[] hBytes = hash.ComputeHash(tBytes);
StringBuilder sb = new StringBuilder();
for (int c=0;c<hBytes.Length;c++)
sb.AppendFormat("{0:x2}",hBytes[c]);
return(sb.ToString());
}
oh yeah, and don't forget to use
using System.Security.Cryptography;