Hi The dell optiplex-780 is a desktop. You can buy a USB wireless adapter from online, Radio Shack, or any computer store. Once you attached this adapter to your desktop then you can use Windows 7 to go on the internet using your wireless account. Using a desktop to go wireless on the internet is unusual but not impossible, it just take more work. Good luck.
Btw I have two desktops with win 7 and use Wi-Fi on both without any issue from day one of the installation. Yes I did have to put in the wifi password, but didn't have to use an Ethernet cable duRing initial setup. Possibly due to the drivers were included with the component when I purchased it. Since you have a refurbished dell, I can see your dilemma. You could contact dell give them the serial number of your machine and get the proper setup from them. Good luck.
I did a quick google search of the PC the desk top model doesn't come with a wireless network adaptor, it only has a wired ethernet connection in the back. If a wireless adaptor was added use window control panel to check look for wireless connection option and configure you may need the access code to connect to the wireless network use the window's wizard to help you configure.