HostedNetworkStarter is a free tool which can turn your computer into a wireless hotspot.
On first launch the program asks you to choose a network name and key. There's an option to share your internet connection (turn this off and visitors can only connect to you and each other), and you're able to set a maximum number of connected devices.
Click the Start button, and within a few seconds your hotspot should be live. A simple console displays basic details on the network, the number of connected clients, transmission speeds and more.
If the "Hosted Network State" stays at "Unavailable", there's a problem. Scroll down the list of network details, and any error messages returned by Windows appear at the bottom.
You can use any variable inside HostedNetworkStarter.cfg as a command-line parameter in order to change the settings of HostedNetworkStarter, for example: HostedNetworkStarter.exe /NetworkSSID "MyNetwork" /NetworkKey "fh78fh8f#"
There are a lot of similar tools around, but HostedNetworkStarter is smaller than most, convenient to use, and displays plenty of information about your hotspot.