Why is this one computer so slow?

Two things to check before anyone spends money:

• Is it a spinning hard drive or an SSD? Open Task Manager, go to Performance, and look at the disk. If it says HDD and sits near 100 percent while the computer crawls, that is your answer. Replacing a mechanical drive with an SSD makes a five year old machine feel new for a fraction of the cost of replacing it.
• What starts with Windows? Task Manager, Startup tab. Years of installed software each adding themselves is a real cause, not a myth.

Two things that are usually not the answer, despite what people assume: the machine being "full of files", and needing a registry cleaner or optimization tool. Those tools range from useless to actively harmful, and we would ask you not to install one.

If it is slow only inside one application while everything else is fine, that is a different problem and points at the network, the server, or that application's database rather than the computer.

A machine that got slow suddenly, rather than gradually, is worth flagging. That can indicate a failing drive or an infection, and both want looking at rather than living with.