SysInternals offers a boatful of outstanding freeware for Windows. My favorites are great for reverse-engineering file and registry access:
- Filemon traces all access to the file system, allowing you to see exactly how an application is using files. Yesterday, it proved invaluable in determining exactly how Lotus Notes finds its notes.ini file.
- Regmon does the same for the Windows Registry, showing all accesses as they happen.
There are other awesome tools there, with source included!