Have I got this right. You are simply replacing your present desktop with another one but will use the monitor attached to the old one? This is straightforward. You disconnect the ethernet cable and the monitor connected to the old desktop and use them for the new one.
You will of course require an operating system before you can use the new desktop and there shouldn't be a problem. You are allowed to reinstall it providing it is only on one machine at a time. You will use the same product key as before - Microsoft would probably only insist that it is reactivated if you try to reinstall it an excessive amount of times. Do you actually have the Windows 7 Disc? When you install it you will need to make sure that it has the relevant updates.
You will need to load the operating system - you may find this usefulenter link description here
Do not put the ethernet cable in until you have loaded the operating system. You will need to set up your internet connection:-
Click Start and select Control Panel Select "Network and Internet" Select "Network and Sharing Center" Choose "Change Adaptor Settings" from the left-hand menu Right click on the "Local Area Connection" icon and select "Properties" The Properties window should display a ticked item - Internet Protocol Version 4 (TCP/IP). Highlight this option and select "Properties" Ensure the following are both checked: Obtain an IP address automatically Obtain DNS server address automatically Select OK then OK again. Open browser.