Disk used to 100%?

Why is my hard drive being used 100% in Windows 10 even though I don't install anything or do anything similar. This is almost always the case and I think that's why the laptop is so slow. It sometimes takes several minutes to open a program.

You just have an extremely slow hard drive in the laptop. An upgrade to a 2.5 "SSD is possible in most cases.

I doubt that it is due to the quality of the hard drive. The laptop is now nothing special but at 500 euro a decent hard drive should be a matter of course

500 euro is lower middle class at best, and the read speed of conventional HDDs is limited. And any exotic server hard drive with 10k revolutions will not be installed there.

There are also plenty of laptops that you can buy for 500 euro, but they are much worse than other laptops in the same price range.