You can't always determine what's slowing down a PC, at least not easily. But taking the following steps in the order I list them will likely locate and solve the problem.
1) Scan for malware. A program that means you no good just might be the culprit. If your PC is mass-mailing spam or taking part in some other evil activity without your knowledge, that could slow it down. I assume you already have a good antivirus program, but just to be safe, install, update, and scan your PC with either SUPERAntiSpyware or Malwarebytes' Anti-Malware (or both).
2) Look for a hog process. A process is a program or an independent piece of a program, and any time you're running Windows, you're running a lot of them. A big, fat, slow one could be hogging all of the resources and slowing everything down.
3) Remove autoloaders. Most PCs have too many programs that load automatically with every boot. (In fact, most PCs leave the factory with too many such programs.) Each one of these slows your PC and may cause conflicts.
4) Keep notes. If the above advice doesn't solve the problem, keep a pad of paper and a pen near your PC. When it slows, write down what applications are running and what you were doing when the problems start. After a while, you may figure out what program or practice is to blame. (Why a paper and pen? So you don't have to load an existing word processing document and start typing while your PC is moving like a tortoise dragging the ultimate gamer's desktop PC.)
5) Change your work habits. I know you don't want to read this, but perhaps your PC isn't powerful enough for what you're doing with it. To keep it moving nicely, multitask less. When possible, close one big application before opening another. Replace your slower applications with faster competitors (Replacing Firefox with Chrome did wonders for my laptop.) And don't upgrade major applications unless you absolutely have to, the new versions are always slower.
6) Upgrade your hardware. If your PC is just too slow for the work you need, or want, to do on it, maybe you need an investment. And no, you don't have to buy a new computer. Adding RAM will give you the most speed for the least outlay.