Michael,

You can remove a file association using the ASSOC command from the XP command line.

Click Start then Run. Type command in the box and click Open.
You'll get a command line window.
In the window, type
ASSOC .ubx =
then hit return.
Type exit to close the command window.

You can see all the capabilities of ASSOC at
http://www.ss64.com/nt/assoc.html
and all the XP commands at http://www.ss64.com/nt/

Mitch