How can I speed up my computer?

I think that my experience is typical. You get a new computer or laptop and it’s pretty zippy at first. But over time, it starts to get bogged down by all sorts of stuff that gets inadvertently installed, even if they are technically not regarded as viruses.
Under Windows, Task Manager shows me dozens of “processes” and “services” running in the background that I have no idea what they’re for and if they are even necessary.
So, what is the best way to clean things up, so that my PC can be speedier? Is there reliable and safe software that I can use?
Is there a website that shows what such things like QWAVE, Mcx2Svc and hkcmd.exe are for and whether they could be safely disabled or removed?