it sounds like you are describing a brace at the corner that angles at 45 degrees that goes from the top of the wall to be bottom, if this is correct the simple answer is NO you should not remove it.
this is lateral bracing for the wall that resists both wind and seismic loads. If you are able to create shear walls that are equal in strength or will provide the required strength necessary for the wall then it can be removed. The building code has a requirement for wall bracing.
if seismic loads, lateral loads and shear walls means nothing to you then you'll need a professional engineer to evaluate your existing home and determine what you'll need to do to remove the lateral brace.
if you can move your opening you should leave the brace as is. Hope this helps! now with that said some photos would help to confirm what you actually have. you also may want to edit your profile to include your location as many times location determines answers to your questions.
"You get what you pay for, and sometimes free costs more!"