If your users have InfoPath 2003 and you are designing forms in InfoPath 2007, save the form as InfoPath 2003 Form Template (in the Save as type). You can also click on Design checker in the task pane and then click on Change Compatability Settings. Then there is a checkbox that says "Show report on computability with InfoPath 2003". That way you can make sure that your template will be compatible with IP 2003.
If your users don't have InfoPath at all, I'd recommend browser-based forms as the next option.
Hope this helps.