It all depends on how in-depth you want to get with the work.
If you are just going to change things like tint, tone or sharpen and blur photos, then a program like PaintShop Pro or Photoshop Elements may be just fine.
If you are going to get into big layouts, image compositing and heavy manipulations, then a full (professional) version of Photoshop is about the best there is.
It does have a steep learning curve at first, until you get to that 'a-Ha!' moment, then it opens up. The other thing, is that it is expensive. Where Elements and PSP can be had for around one hundred dollars, a full copy of Photoshop was over 600 last time I looked.
If you can find it used, Photoshop 6 which came out about 6 years ago, will run on XP handily and it has all the major filters and functions that matter. It's got all of the muscle and none of the 'fluff' that the lastest version's got in it now (the Adobe software engineers *have* to keep themselves employed, after all!). I have a Mac and am still running PS5.5 to do the bulk of my own image manipulating and editing - the program is quite powerful, even when it's as old as what I've got!
Yeah, I use Photoshop daily - check out my teeny-weenie avatar! - I 'manipped' that with Photoshop.