DropMyRights
It can be a chore to use a limited user account in Windows XP. It is an even bigger task to get other people to run as a limited user. In the past I recommended MakeMeAdmin as a way to take care of administrative tasks while still logged in to a limited user account but, DropMyRights seems to be a better way to go.
Instead of using a LUA, DropMyRights starts internet aware programs (or any other program) with limited rights, so It is possible to use an administrator account as your primary login and run email clients and web browsers as a limited user.
After downloading, run the msi installer and then copy DropMyRights.exe from where it was installed to somewhere in your path (C:\WINDOWS in my case) and then uninstall it (all the other files are source code and the EULA).
Right click on the shortcut for the application you want to run with limited rights and in the Target field add C:\WINDOWS\DropMyRights.exe.
So:
"C:\Program Files\Mozilla Firefox\firefox.exe"
Is now:
C:\WINDOWS\DropMyRights.exe "C:\Program Files\Mozilla firefox\firefox.exe"
Edit:
There is a DropMyRights clone called StripMyRights. It does the same job with some modifications like passing command line arguments and the ability to be called from the registry.