Here is a short answer to your question. Windows 7 is quite reliable and very effective and most users do not have the kind of problems you are experiencing.
Rather than suspect your operating system, why not consider a possible hardware problem. Memory errors are not always flagged by Windows. Also, some types of hardware errors are not easy to diagnose. One of the tools you should have in your resource Kit is some kind of bootable USB or CD that will test the memory. There is a version of Mame test 86 that will do that.
The other problem that most often slowed the computer down is some difficulty with the disk drive. Again, way to check this quiz with some kind of diagnostic tool that focuses on the integrity of the disk drive. If you have a Seagate drive you can go to the Seagate website and find a tool for testing Seagate drives. Same goes for Western Digital, they also have a testing tool for their hard drives.
You did not specify what kind of computer you have so it's difficult to guess what sort of problems are frequent with that brand or make of computer unless we know what the make and model is.
