Thought I would pass this tip along.  I recently setup my parents computer with Open Office (WinXP too).   My parents needed basic Office functionality, I was impressed with the Open Office suite.  My Dad uses the envelopes feature the most.  They had a HP 940 printer, which would handle printing envelopes.  I was getting inconsistant results when getting it to work with one printer configuration.   

What I wanted to pass along for others looking at Open Office, I created a 'default printer' option with regular settings.  I created an additional printer setup to support Envelopes, which was pointing at the same printer.  So inside Open Office, when I wanted to print envelopes, I chose the 'envelope printer'.  This worked liked a charm.  I got the idea from a post here

Hope this helps.

Steve