You'd have to go as far back as LR4 and Elements 12 for Vista I'm afraid.
There is the GIMP which isn't so fussy about minimum requirements. They even talk about being able to use a P4...
I can't find any downloads for the full version of LR4, only updates..
Is GIMP any good and can I get/ try it free?
The "updates" are actually full versions: you can use them as trial versions for 30 days. The latest version is here: (not the same link as above) Adobe - Lightroom : For Windows : Adobe Photoshop Lightroom 4.4.1
GIMP can be found here: GIMP - The GNU Image Manipulation Program
It is quite good, but only if you have never used PS before, but that's more from a user point of view rather than the software itself. I've tried it and found that because some things (such as shortcuts, or ways of doing things) are slightly different it's rather annoying initially. But otherwise, for free, it is very good.
While you would be able to use the trial version for 30 days, at the end of that trial period there would be no way for you to purchase it from Adobe because Adobe only sells the current version. You would have to find a third-party seller who still has Lightroom 4 available.