I want to see how well that IDE runs on my old Windows XP laptop.
I looked on www.lego.com, but it seems to all be documentation (downloads).
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The Mindstorms NXT GUI, NXT-G, is distributed on a CD and is not available without the Mindstorms NXT kit. It's not explicitly stated, but you can think of the license included in the price (although what the actual licensing terms are I don't know). In any case LEGO does not offer it for download. (Edit: I was so wrong - I'll just leave the rest of this answer for the BrickLink reference).
If you want the actual CD, your best bet to acquire one is to get one from someone who doesn't need it. For example, someone who happened to buy two kits. There are some available on BrickLink as well.
You could also try to contact the LEGO customer service and see if you can get it straight from them (mention the item ID 4558460, it should help them find it) as well, but I suppose they'll just point you to the download.
As of January 2017, the links in the older answers to this question all seem to be broken. However, there is still a download available for NXT 2.0 on the LEGO customer service website. This link seems to have withstood many years of changes to the LEGO website: http://service.lego.com/en-us/HelpTopics/default.aspx?questionID=2655.
Look for "How can I get a replacement for the LEGO® MINDSTORMS® NXT software for set 8547?" on that page.
There is also currently a page on the official Mindstorms site for the NXT software download. I'm not sure how long the link will last, but here it is: https://www.lego.com/en-us/mindstorms/downloads/nxt-software-download.