If you've downloaded Safari for Windows and are a looking for a tool similar to Firebug or Nikhil's Web Development Helper, then you might be interested in the WebKit Open Source Project. Within the project is a tool call Web Inspector that performs much of the same tasks as the tools mentioned earlier. To run the tool:
1) Download the nightly build from http://nightly.webkit.org/.
2) Extract the contents from the zip file.
4) Execute the run-nightly-webkit.cmd file. This will launch Safari from the command prompt.
After Safari is launched you will have an additional item in the context menu called 'Inspect Element', select this to see the tool:
Here are a few screenshots of it in action: