Encrypting Large Files

OK, so I can encrypt small files using Des Encryption, but whenever I try encrypting large files, The program's memory usage skyrockets! Would anybody be able to get me some code that will let me encrypt large files (It doesn't have to be Des, but I need to be able to support any length password)

Could you show your code? Maybe we can see why it's 'skyrocketing'. Anyhow, you can check the encryption class I created, check my signature. That's for encrypting strings, but you just have to change the filestream source so it'll encrypt a file.