If you're using Windows Vista or Windows 7, are you able to view Word documents within Explorer's viewer panel?
If you're using an earlier version of Windows and have Outlook installed, see if you can email yourself a .doc attachment and then, when you receive the email, view the attachment within Outlook itself by single-clicking on it.
With Office 2007, Opus should be using the preview handlers that are installed with Office, the same as Outlook and (in Vista/Win7) Explorer use. It sounds like the preview handlers are not working for some reason.
Check that the Opus ActiveX + Preview + Office + Web plugin is enabled (Settings -> Preferences -> Plugins -> Viewers) and then select it and click Configure. You should see something like this, with the MS Office preview handlers near the top:
Check that the MS Office Word preview handler is listed near the top, and enabled, with the appropriate file extensions (.doc, .docx, etc.) assigned to them.