If your current desktop is working okay, how about upgrading your system to Windows 10?
You can buy a copy of Windows 10 from Best Buy for around $139.99. Alot less expensive than a new desktop.
Hope this helps.
Keep your old computer and use it to explore the world of Linux OS. You should replace the old hard drive with a 250 or 500 Gigabyte Solid State Drive. That old drive will probably fail soon anyway. Make sure your present computer hard drive has a SATA interface before you purchase an SSD. Then install a copy of Linux Mint 18.3 or Linux Mint 19.2 which you can download for free. Back up all your data to a flash drive or external drive before you replace your old one. Then copy all data to new OS and to your new Windows 10 computer.
Mint 19.2 will be supported until 2023. Just do weekly backups as needed to keep the machine secure. No other antivirus or cleaning software is needed. Both come with Firefox, and Google Chrome can be easily added. All software is automatically updated each week. Office documents can be viewed and used in Libra Office which is also installed free.
Windows 10 is quickly moving toward compatibility with Linux. Make sure your new computer processor has the capability to handle Virtual Machines. Google WSL1 and WSL2 and watch some of the videos on this subject to get an idea where Microsoft is heading with Linux. A Fuller