How can i partition an external hard drive to use it for both windows and mac?

Okay, since I prefer windows I'll give you instructions on how to partition your 1.5 TB drive on Win XP. Right click "My Computer" once your machine is up and running and click "manage". Click on "Disk Management" on the left side panel and you should see your primary drive listed and your 1.5 TB drive. Right-Click on the 1.5 TB drive and click "Shrink Volume". Select the size that you want and continue. When that is completed you'll have raw volumes that you'll need to format. The 500 GB volume format as NTFS. To do this, right-click on the 500GB volume and click format. Select the appropriate options, give it a name if you wish, and proceed with formating it. Once you're done with that, connect your drive to your Apple computer and navigate to disk management. (That should be in system properties if I recall). Format the 1TB side of the drive in Apple's trademark Journal system and it will be recognized by your mac for full use. Connect your 320GB drive to the Mac and do the same. Now you have a partition free for windows xp, 1TB formated for use with Time Machine, and a 320 GB drive for whatever you want. That should just about do it. There are applications available that claim to make things easier for you, but really the tools you need to format and partition drives are included within the Mac and Windows operating systems and therefore it really isn't necesary to have something with additional features unless you wanted to do some added options with formating. Good Luck