In your IT estate: How many computers do you have? What about printers? How many copies of Windows? What service pack level are they? What do they connect to?
If you are planning to take the business forward then you must know where you are starting from and there resources you already have available.
Therefore as part of a forward planning exercise you should set up and maintain a configuration management database (CMDB). There are several tools that can be employed to assist with keeping this up to date but for most modest environments it can be updated manually as change happens. This can also be used as an asset register and a tracking system so you know the state of your estate by simply querying the data.
This becomes critically important when you are assessing the impact of an outage, working out where the single points of failures in a service along with planning your next change.