AirPort Utility will not do anything to help you, even if you can install it on the PC.
You need backup software for the PC that will recognize a network drive......like the Time Capsule. The software that you have been trying to use will only work if you have a USB hard drive connected directly to your PC.
Frankly, it is much, much easier for the PC to back up if the hard drive is connected directly to the PC, as a USB drive would be.
I'm not recommending that you attempt back up the PC to the Time Capsule, but if you want to try anyway, take a look at the following software which is capable of backing up a PC to a network drive.
http://www.macrium.com/product/7/macrium-reflect-v6-home-edition.aspx
Genie Timeline Pro | Overview | Time Machine Backup ...