First off, let's assess what you're trying to do.
The subject of this research is old (maybe obsolete) software. What you're looking for in this software is answers to "what patterns to drop", and "what patterns to keep".
Since users take good patterns for granted, as they should go unnoticed, you probably will want to focus on what the bad ones are.
Don't see this as "making fun of this old piece of software"! You'll have to make sure your users mold their negative feedback properly though. "Feature X sucks!" doesn't help. "Feature X didn't behave as I expected it would" however does help you.
It is imperative to set a challenge to your audience that allows them to focus their feedback (and if the software is exceptionally frustrating, keeps their spirit up as well!).
For example, you could precondition their experience with "This piece of software is rubbish. We know that. But we'd love you to have a look at it and define why certain things are bad. The why and what are more important than the fact that it's less-than-ideal."
TLDR: Acknowledge and address the inferiority of the product, and help your guinea pigs to focus on reporting useful feedback.
The form of the feedback (open questions, multiple choice, etc) depends on the size of the audience and the reporting tools at your disposal.