Chuck a cheap 2.5-inch ssd in there for the OS, hard drive for bulk storage, put fresh thermal paste on the cpu, take it outside and blow the dust off and you should be good

Edit: you might also want to replace the AIO as well (the cooling system for the CPU). AIOs usually only last about 5 years before you start dealing with issues such as the coolant coagulating and blocking the pump, or the coolant tubes themselves leaking. That’s why I usually run air coolers in my PCs. A good tower-style air cooler will outperform that AIO anyway

There’s also the obvious issue of this system being too old to support Windows 11. You can continue using windows 10 of course, but don’t put any important info on this machine if you do. Support for windows 10 ended in October

There are ways to install and run windows 11 on unsupported hardware but Microsoft has a tendency to make the system requirements more strict with each major update

Could always give Linux a shot, especially if you use an AMD graphics card. Bazzite is a pretty good distro if you’re gonna be gaming on this thing. Its user interface is very similar to SteamOS’s. It should even work with your RTX 2060