J,
The easiest way to solve this is to use the gd dll that comes with your PHP distribution.
In <PHP4 Home>\extensions\ directory you will find a file called php_gd.dll.
To enable all the GD functions, simple uncomment the extension=php_gd.dll line in your php.ini file, bounce Apache et voila.
Cheers,
Justin
P.s there are a couple of very good tutorials around drawing graphics in the Articles section of this site. There are also quite a few already written PHP libraries for graphing.