Can a wired computer be adapted to use wireless?

If you want the desktop to be on wireless, you can buy a wireless adapter for it. It will still work fine over a wired connection along with wireless devices, though.

You will need a Wireless Adapter that connect to a USB port You can use both a Wired and Wireless connection setup at the same time ( I do so when I go to bed at night I turn off the Wirless setting on my router, I don't know if these Wireless signals interfere with sleep patterns but I'm not going to find out either also extra security, I only need the Wireless for my portable devices when I'm in another room)

It won't affect your computer, the computer uses a different connection up to the limit of connections on the router.

Yes