The problem with remote desktop is that it blanks the remote screen so you can't use it to show the other person what you are doing.
It also needs the account to have a password and have a few security settings turned on, it isn't included in the Home edition of XP.
Also remote desktop and any of the regular VNC tools require you to know the other persons IP address and have them forward a port on their router (not granny proof)
There are a few VNC versions that call out to a 3rd party server who makes the connection between you and the person you are helping. Some of these are free, one very good commercial one is https://www.copilot.com/, it's about $5 for each day of usage or free at weekends.
From a pure usage/reliability point of view, of the ones I have experience of, in order of preference:
Hyena (RealVNC) - you can also see if a person is logged on before attempting to connect
DameWare Mini Remote Control - as good as Hyena except you can't initially see if a person is logged on, also saves connection settings
Remote desktop Connection - only third because of the "blanks the remote screen" already pointed out, we only use it on servers
Gencontrol - not as reliable as the first three but does the job
the other posters have covered everything i'd bring up
one thing i'd like to add:
please ensure you use strong passwords for any kind of remote access. Im not sure how many of you actually have ever taken a look at your computer's security audit logs but on mine (i use windows remote desktop) i usually used to have anywhere from 1000-2000 failed bruteforce login attempts. when i found out, i started using an open source solution that ban's the remote ip address after x number of failed attemps within a given time frame https://github.com/jjxtra/Windows-IP-Ban-Service
This is meant to work with windows remote desktop
general tips to be safe:
use a strong password
disable the default administrator account on your system (or at least rename it)
dont use first names as your account name (bots try first names as usernames)
dont allow a user to have admin rights when connecting remotely
If the target computer is Windows 7 (or 8) Professional or Ultimate, Microsoft's Remote Desktop is the best option. It is secure, stable (and free if the named versions of the OS). It does have to be enabled, and your logon has to be specifically given to it before you can connection from a remote location.