People could probably help you out a lot more if you could specify which types of files you are trying to open, along with which program you are using to read your email, or if you are using a web browser, which version of OS X, etc. For example, if you're using the latest version of Apple Mail, all you need to do is click on the "quick look" button and you should be able to view most attachments. Certain file types (e.g. .wmv files) may require that you download and/or purchase additional software. If you provide more details, the people on this forum can be more helpful.