I'm not sure which Operating System you're using, but on most Windows e-mail clients (such as Outlook and Eudora), you can simply drag the file from your folder onto a new (outgoing) e-mail.

As for attaching e-mails over the web, that really depends on what you are using. Hotmail for example requires you to press a specific button, which allows you to browse your PC for the file you'd like to attach. Other sites only allow you to enter text, no attachment.

But you said that people couldn't open it. Did they mean they received it, but couldn't work with the file? Perhaps the file format is something they can't use (.exe files won't run on Linux for example - not natively at least).