Need two desktop computers, one motherboard just died and the other is plagued by viruses. The cheaper the better, preferably $400 or $500 or less. No monitors needed. Both will be used for internet 99% of the time. Links would be great, not building it myself. Very satisfied with the monitor you guys recommended a couple of months back. Thanks for any help.
Refurbished Mac Mini from Apple Store.
I had Dells for many years. They were big, they vibrate, were loud, and plagued by viruses and blue screens. Switched to Mac Mini a few years ago and never had a single virus attack. Whisper quiet, beautiful to look at, takes up very little room. I also get free OS upgrade from Apple. It is perfect for Internet access, with occasional word processing and printing.
I mentioned refurbished to keep the cost down, feel confident to buy from Apple directly. Add MS Office for Mac for about $100 (3 licenses) if you need it.
So you change the computer because it is plagued by viruses?
I built an intel nuc last october for around $500
Its very capable for most day to day tasks and even a little gaming 
I got the i5 model
yes, OP realize that unfortunately most pc problems are the user.
Remove the virusses, act to avoid future infections.
Hitmanpro http://www.surfright.nl/en/hitmanpro
used first time is free (you have to give emailaddress if I remember but no spam)
Need it more then the first free period, pay for it, it is worth it.
After cleaning, winpatrol, free edition it warns when something changes in for example programs that start with windows, if the warning comes right after you installed something it is cool, f not, action is needed.
Use and update the free security software from Microsoft, it is their OS, they better help protecting it.
Do not use more virusscanners at the same time
when installing free ware, check the install process, a lot of those try to add crappy stuff like browser add ons, mcafee virus scanner, other software.
Keep your flash and java updated, notorious for holes in safety.
Better: NEVER use Adobe Flash &/or JAVA unless absolutely imperative. 
Best Regards,
George
true, but you gotta love Minecraft
Format the drives in the virus filled computer, install a fresh copy of windows (not a downloaded BitTorrent version) and install a good antivirus. Problem solved. If you don’t have a legitimate copy of windows (and don’t want to try Unix) then your cost is a windows disk. I suggest Windows 8.1 with Classic Shell.
Tried to download hitmanpro and was told my firewall might be blocking it and I needed to adjust my firewall to allow the program to run. Have no idea how to do that, any advice?
If you’ve got a W7 COA sticker on your rig you can download a new ISO from microsoft. I’m assuming it’s a W7 pc.
My advice on this topic is still the same as I offered in the other recent thread. The OP of that thread rolled with the suggestion to purchase a particular used PC for under $200 shipped, then add a Samsung SSD. Total came to under $300 for a rather capable PC that included 8 GB ram, a W7 license, and a decent graphics card.
Indeed, do download it from a clean computer
Microsoft also has a free antivirus program, if you have genuine Windows 7(?) and above
If a machine is riddled with viruses then the only really safe thing to do is to format and reinstall. You can remove stuff, but after 20 years building PC’s and servers it’s often quicker and easier to save the data and reinstall.
The next thing to do is to rethink how you are using your computer. Windows doesn’t automatically have issues with viruses unless the operator is making mistakes. Use AV (Windows Defender is fine), install Microsoft updates on a regular basis (I manage updates for thousands of computers as part of my job, issues caused by updates are rare these days), install updates for 3rd party programs on a regular basis, consider paying for Malwarebytes as an additional layer of security, use an adblocker everywhere and when running and downloading software be careful.
+1