As options vary by make and model, I've moved your post over here to the Desire Support forum.
If you're not rooted, and if you haven't installed a custom dialer, and if this isn't a known problem for the Desire (you'll soon know by community response here), then my personal advice would be to back up what you can and perform a factory data reset. There are various cache areas (just like with your web browser) where things can get tangled and go wrong - for unrooted users, factory data reset is your best bet to clear that sort of thing up. Many find MyBackup Pro to be a good backup tool.
Hope this helps!