OK. So, you haven't installed it yet, correct? Google is your friend. I did a quick search for "how to install linux mint dual boot xp" and came up with several very good tutorials. Have you tried that? I haven't actually used mint, so I can't get very specific about it. Unix/Linux has what is known as a root user. It is similar to the local administrator account in Windows. From you normal user account, you can use the su (substitute user) command to switch to the root user to run commands that need administrative privilege. There is also a command called sudo. If sudo is set up correctly, from your normal user account you can run "sudo command" to run a command that normally would need root privileges.
Does that at least get you pointed in the correct direction?