If you want seemless integration with Thunderbird, I think you're going to have to use the enigmail plugin. I like the portable version of thunderbird with enigmail from portableapps.com. You can use truecrypt to encrypt a usb drive and store your thinderbird client. That way, your private keys are protected. I used that setup for a while. I'm now actually just using the FireGPG firefox plugin with my gmail account. Its a nice plugin and makes encrpyting gmail very easy. You have to install GnuPG for the FireGPG plugin which you can use to encrypt files. For file shreding, I use SDelete from the former Sysinternals which you can get from microsoft now. Link here.