Without meaning to sound catty, Siri is really nothing more than a search engine hooked up to speech recognition software. So building it for WinXP would be without any real merit.
Also, Siri is of immense scope and sophistication. If you have to ask "how do I design a computer program" you 99.99% do not have the capacity or resources or skills or knowledge to even attempt something like Siri. You are years away from being where you need to be.
If you want to learn to code you need to set your starting goals much lower. Siri can be your inspiration but it should not be your first objective.
Not that you will reply, but this smells like trolling...
as wombat_tg pointed out consider that there is really only one Siri, developed by a company that has essentially unlimited resources. If you have to ask "how do I" it is so far out of your element that it's laughable. Similar to asking "How do I convert my 1985 Escort to take me to a shuttle to take me to Andromeda, and I need to be back next week for a birthday party."
In the remote chance that you are not a spambot or a troll, and that you will actually read this again, I'll try to be constructive. Purely out of curiosity I looked into voice recognition libraries for VB.NET. I used them to understand when I spoke a color, then it changed my form background to the color that I spoke. I suggest that you start out with something like that. Get VB.NET express (free) then google "voice recognition for vb.net". I'm sure you could even find a youtube video that will get you going.
Once you complete that, then you should be able to get your Escort converted.
This a bigger project than a few lines of code. The first place to start would be making a database with replies and questions. Then recognizing what they said, and trying to find it in the database, next you would just use the reply to that question, or statement. This would be a very difficult project for a beginner.