In order for a computer to be wireless enabled, it needs to have a wireless network interface card. With desktop machines (which sounds like what you have because of your use of the word 'tower'), these are almost always separate cards added to a free PCI slot on the motherboard. With laptops, they are almost always built in. A while ago, they used to be mostly PCMCIA, but that's very rare these days...
Where did this computer come from? Generally speaking, desktop machines do not come with wireless NICS, because they don't need them. They sit on the desktop, so the are wired directly into the network by means of an ethernet cable.
Obviously, opening the machine up would reveal straightaway whether it had a wireless NIC installed. However, I'd never advise a novice to do that.
An easy way to tell would be to look in Device Manager and see what networking devices you see... How you do this, will depend on what version of Windows is running on the machine. Obviously, it's not running Windows, that's a different matter, but still not a problem...