If you open an email attachment directly from an email and then edit it, you must save it correctly and securely. If you just click 'save' the file is stored in a temporary Windows system folder which is lost when you log off.
There are two safe ways to work with email attachments:
1. First save the attachment into your My Documents folder. Then open the file and edit it.
2. Open the File in Google Docs and edit it using the online editor