win xp users ??

I have win XP no network connected just 1 box 2 users
admin. (root) and then a regular user. I don't really understand how all this works. like data storage and stuff

when i install a file, under admin, it is like not there under the other user, and vise versa, i am sure it is a setting that i have

the deal is this.
i do allot of stuff all the time that requires some admin prevliges
however i don't like running under admin cause the bad people can hack in and take over more control when under admin

so i am trying to understand how it works, the whole user set up thing, i have looked at several windows books and none of them explain it very well