Is simply deleting "domains" a very bad idea? I'm sure it doesn't take up too much space and I see kite related info and other history related to browsing I still do today but I really don't care about the early developmental stuff we looked at with or for my kids as they grew up and now that they use the computer they load more stuff on it than I can keep up with.
Don't delete 'domains', but all of the sub-data to domains can be deleted. Unfortunately, I looked in my own reg, and there was no info under the heading domains, but if you click 'domains' on the left, and there's a list a mile long on the right, the only one you would have to leave would be the first one called 'default'.
Making a restore point before you do this is always a good idea, and backing up your important data is a good idea, too. Not that this particular reg poke is going to kill your data, but it is a good idea, especially with an aging machine.