One way to find out what is or may be happening is to launch the performance monitor and the resource monitor.
Look at everything that is running and how much load is on the system. See what is dragging down or slowing any particular application or service.
The Event Viewer logs will tell you about errors and warnings that occur.
Takes some time to get a sense of what all that information is really telling you. Do not get overwhelmed and just explore it all a bit. You will learn a lot about your system in the meantime.
But if you look at and address the biggest offenders (i.e., programs, services, etc. taking CPU time, memory, lots of disk activity, etc.) you can start fixing them one by one. Just google for more information...