windows xp documents and settings folder question

ok I am not a total nube to computers but there are some things that I am not sure about, for instance I am the only one that uses my pc I have turned of the secondary login but when I look in C:\Documents and Settings I see other folders like ADMINI~1~000,SIR , NetworkService.NT AUTHORITY.000 , NetworkService.NT AUTHORITY, all users, default user, local service,
some of the folders look the same only with different dates on them. I am wondering what all of these are and if I can get rid of some of these because when I look inside of the folders they seem to be the same and my disk space (120 GB) is at 80 % but I don't really have allot of vid or other files that would take up that kind of space