I have tried a number of plugin type PGP programs for Outlook and all of them have been less than reliable.
I found this PGP program called WinPT. It runs by itself and makes signing and encrpyting e-mail VERY easy. It also has the ability to encrypt files in a number of formats.
Here is the link:
If anyone gets stuck generating a keyset or just getting started altogether just send me a message and I will post a Step-by-Step for this little tool.
Hope this helps!