so I guess what I'm asking for is, what do I need to know about the XP file system in order to be able to write a few simple scripts to "safely" overwrite (ie, temporarily creat enull files in the place of) essentially all non-used blocks on my hard drive

If you're just interested in writing something to all the unused blocks, why not just use them: create a file, then keep appending pseudorandomness until the drive is out of space. For added assurance, seek back to the beginning and write different pseudorandomness over top of that; repeat as your paranoia level requires. When you're done, delete the file. You may find that defragmenting will speed up this process.