You can use a PC with a distro like ClearOS installed and configure it to be an AP. This will be tricky. Instead, try something simple first. Get yourself a wireless N router that supports the maximum Wireless range. Set the router in the middle of the building and have all PCs connect to it. You can also use multiple routers that are wired to the central router so you can cover more range.

AP = Access Point

There is a tutorial at http://www.linux.com/archive/feed/55617 which will guide you to make a Linux based system with a wifi card a Wireless Access Point.

However you must consider a few things with this setup, The quantity of users and the range. The quantity of users can be an issue because most "home" wifi access points only have enough memory for 20-50 attached devices. In addition the quanity of users also effects the throughput of each antenna, having 100 users on one antenna would make it horribly slow.The ranges for each wifi type are listed at http://en.wikipedia.org/wiki/IEEE_802.11#Protocols, most likely one device will not cover the entire building, you will need multiple WAPs or to use wireless extenders (http://www.newegg.com/Product/Product.aspx?Item=N82E16833995002) in strategic points.

Based upon my interpretation of the network the best approach would be to setup a mulitple WAPs (wifi routers) about 100 meters apart, this would limit the users per WAP to fit the limitations of the router and reduce the load on each network device antenna. In addition if you plan to service that entire building with a single internet connection you can also run into bottleneck issues, you will need to consider getting multiple ISP connections coming in to increase the throughput per user.