The Spirit control is an ActiveX control that is distributed and installed by Mindstorms and CyberMaster products. It is used as an interface between the PC and the RCX brick. The RCX contains (by default) software that runs a byte code interpreter. The Spirit control makes it relatively easy to download instructions to the RCX which can subseqently be executed by the interpreter to make your robot do what you want.
LEGO has documented the Spirit OCX as part of the LEGO Programmable Bricks Reference Guide . Using the information in this guide you should be able to drive your robots from a variety of Windows programming systems such as Visual Basic or any applications supporting VBA, such as Microsoft Word.
Note that, because the Spirit control is a Microsoft style OCX control, it will only work on Microsoft Windows based PCs. In fact it also is limited to the later versions of Windows, 9x/NT/2000/XP; it will not work on Windows 3.1.