Can anyone recommend a network auditing software package?

Needs to be able to run on anything from Windows 98 to 2003 server.

If it runs just with 2000, XP & 2003 it would be ok.

Ease of use, distribution & mainly price are the real importances with the software as a lot are licensed per pc which gets into silly money, will be used for a range of 10 PC's to 100 PC's tops.

Thanks