Making an older computer new Options

[Maccabee]

I am currently on a desktop running xp(my favorite os) and it used to be somebody elses and I had a few questions.

1. How can I remove programs properly without leaving regisrty files and junk which eventually slow it down.

2. How can I change the full computer name.

It was music box but i changed it to CohenComputer. But music box keeps coming up all over the place. More about that will be explained in a minute.

3. When I go to documents it doesnt show the My videos My music and My pictures buttons.
When open the folder ARCHIVE_THIS_PC I see them with their little icons. I tried to just copy and paste them to my documents which worked but then whne I open the start menu the shortcuts to the right changed from My Music and My Pictures to Cohen's Music and Cohen's Pictures and I cant click them! ANd when I right click them and click properties it says the location is blahblah\Music Box\documents\Archive this pc\my pictures or whatever. Weird I know!

So if I oculd have resolutions for those that would be great or if I could find a way to pretty much reset my computer or format it but keep the os. Cause I dont have an xp disc.