This page offers many tips on using stationery:
http://www.mindspring.com/~majik/docs.htm
Pay particular attention to:"Setting Up Outlook Express For HTML" and "Some
Common Errors You May See".
It's not clear to me what setting you may have changed, but if you work
through these basic steps you will likely discover it.
Another great source of OE help is : http://insideoe.tomsterdam.com/
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  A friend recently re-built my computer and changed my operating system to
XP Pro. When we first fired it up it accepted all my e-mail stationery made
or saved on OE6.
Then, I pressed something and now I cannot create stationery, use many saved
stationeries, backgrounds, or use "Format" to change colors of backgrounds
or fonts. It still lets me use some stationery (All backgrounds are white)
and I can add pictures and midis to the stationery.

               The NOSPIN Group Promotions is now offering
              Mandrake Linux or Red Hat Linux CD sets along
             with the OpenOffice CD...  at a great price!!!
             http://freepctech.com/goodies/promotions.shtml