To avoid viruses launched automatically by viewing HTML messages, is there a way to view all messages as text?
Normally Outlook views all messages in the format in which they were sent in. And there is no feature that will allow changing the default viewing format.
Microsoft KB document Q307594 outlines instructions for changing viewing format to text only.
Enable the "Read as Plain Text" Feature
WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
To enable the "Read as Plain Text" feature, you must make the following additions to the system registry:
Click Start and then click Run. In the Open box, type regedit.
Navigate to the following registry key:
On the Edit menu, point to New, and then click DWord Value.
With the new Dword selected, type ReadAsPlain.
Double-click to open the new value. In the Value Data box, type 1 and then click OK.
NOTE: "Read As Plain Text" is turned on and the BodyFormat property is locked at 1. (Help says it's read/write.) Any attempt to set it to another value will result in an error.
Click OK and then close the registry.