Office 2010 ramps up its attempts to protect viewers from dodgy documents, popping a warning and restricting editing on anything that gets emailed to you. If that seems like overkill, here's how to fix it.
By default, Office 2010 assumes anything you get sent via email is potentially risky. If you double-click in Outlook on an attachment for a Word, Excel or PowerPoint file, it will open in the appropriate application, but a warning will appear on screen and you won't be able to edit it until without clicking on 'Enable Editing'.
That's not unreasonable if you get unexpected documents from unknown sources, which can easily be a source of malware (though that should be picked up by your existing antivirus and mail protection systems). In practice, though, your job might well mean that the only attachments you get are presentations from Bob in the next cubicle, and having to click through that dialog every time can be a nuisance. Fortunately, you can switch it off.
Within an Office application, click on File, select Options, click on Trust Center, and then click on Trust Center Settings and select 'Protected View' in the left column. Make sure 'Click on Enable Protected View for Outlook attachments' is unticked, click on OK a couple of times and you're good to go. (There's also an option to disable the setting for files originating from the Internet, but that's probably best left in place.)
This setting is maintained individually for each Office application, so you can (for instance) enable protection for Word, but disable it for PowerPoint. If you work in a corporate environment, your ability to change these settings may be restricted by an administrator. In that case, you'll have little choice but to click through the Protected View dialogs (assuming you're allowed to do so).