|It's been ages since I used TC or a stand alone DOS environment, so my recollection of either may not be right. Nonetheless, IIRC, you can set the environment temporarily via the command window|
With Windows, you may need to modify the system variables... I don't have XP handy, but under 2K the process is:
right click on the "my computer" icon, choose properties, click the advanced tab, click the environment variables or such button, and then under the "system variables" listbox, edit and append the paths as above. Lastly, reboot for it to take effect.
HTH or at least will inspire someone who does know to help you out. :P