There nothing special about editing xml or ini files, there are just text. Though there are some commercial editors geared toward manipulating xml files.
You could simply get by with notepad.
There are some freeware and shareware text editors out there. I used to have a list of them but I don't have one handy.
If you're looking for a good general purpose editor:
I use UltraEdit http://www.ultraedit.com It's shareware ($30) but its a really good editor with lots of features. I use it all the time.
TextPad is another editor that's simlar to ultra edit. It doesn't quite have all the features of Ultra Edit but its got a slightly cleaner interface. http://www.textpad.com. I think it's around $25.
Pesonally I'd love to have Visual Slick Edit (a high powered developer's editor) but its a whopping $300.
If you have a subscription to StarDock's Object Desktop, it too comes with an editor ObjectEdit.