Sysinternals has a reputation for some of the most useful Windows utilities available. Sysinternals founder Mark Russinovich discovered Sony's music CD rootkit. He did so using RootkitRevealer, his own program.
Many of the Sysinternals programs can help diagnose security holes. Others extend some of Windows' built-in features. Best of all, the Sysinternal programs are free.
Here are a few tools that may come in handy. Note that this is only a fraction of the ones available:
Process Explorer offers more information than Windows' built-in Task Manager. Process Explorer reveals which programs are running in the background. It also lists additional files that many programs load. This can be useful for tracking down pieces of persistent spyware.
AutoRuns shows which programs are set to start automatically with Windows. You'll see where each program's files are located. AutoRuns also lists the Registry entries associated with the startup programs.
SDelete is a secure file deletion tool. Files deleted with SDelete are overwritten several times. The process renders files unreadable, even to recovery programs. Use SDelete to clear out private information if you sell your PC.