The Windows Task Manager can show useful stuff-
The applications tab gives a list of windows open, the performance tab lets you know the amount of work the critical areas are attempting. If the CPU is very busy, it could well be that either there are too many jobs being demanded at the same time, or that one job is just too big. If there is a lot of PF Usage in the history, thats a good sign that there isn't enough memory, and, again, it might be just one program hogging it.
Then there is the Processes tab. This is a long list of techno-gibberish, BUT, there is a column of changing numbers under the CPU heading. These always add up to 100. Down at the bottom of the list is "System Idle", which is normally the biggest number. A persistently large number elsewhere is worth investigating. There is another column under the Mem Usage heading, telling how much memory is being grabbed by each process. Image Name will show lots of SVCHOST.EXE - this is an internal windows program a copy of which is grabbed by just about everything that is running, and is best ignored. The rest will give the clues as to what is actually using the resources for further checking.
Personally, I use a program called "Startup Inspector For Windows" (SIFW), which lets me sort out just what is allowed to run at startup. There is a built in utility in Windows, but I find this much easier to use. I also have a system monitor running called (dont laugh) "Rubber Ducky" by Mimarsinan. Its a postage stamp sized window that displays pretty much everything thats going on in a pictorial manner (the ducky, water level, bubbles, fish, waving weeds and clarity of water). And if you move it, it squeaks like a bath rubber ducky. Both these programs are freebies.