All of the security settings that you will configure are found in the Tools menu -> Options... I will outline the steps in bulleted form based on the tab that you will be in. In Windows Live Mail, You will also look in the Safety Options... menu, under the Security tab.
Tools -> Options
- Read Tab: Uncheck Automatically download message when viewing in Preview Pane. Check Read all messages in Plain Text
- Security Tab is fine unless you have digital signatures in which case you can sign or encrypt your e-mails
- Send tab: If you want, you can force OE to send in Plain Text, but this really doesn't do anything for your security.
Safety Options...
Security Tab: Make sure that Block Images and other external content in E-mail is checked, and optionally uncheck the Show images and external content from senders in my safe senders list.
*Windows mail and Windows Live Mail include a junk mail filter. You can configure this to different levels (maximum puts everything that's not on your safe list in the junk folder, where low or off basically disables it).