Even if you have an immensely powerful machine, the chances are you will have to face that question one day: should you reformat your computer and install Windows once again? Unfortunately, nothing stays great forever and you may have to take more decisive steps. But how can you precisely say that winter is coming?
Once you have reached that point there is really no return and, even though I would love to be kind and helpful, no programs such as CCleaner, ADWCleaner, and others will solve the problem. I believe you will know that you have come to that point, but you may stay in denial, so let me make it clear for you.
It takes forever for your system to boot. This is a real pain in your three letters: you cannot simply press the ON button and count to ten (unless you mean ten thousand). Of course, if you have a very weak machine (such as a typical netbook) you cannot expect it to stand up to the challenge immediately, but still you can notice that the booting time has prolonged significantly.
There is a lot of crashes. Basically, programs can stop responding at any moment, for any reason (or, obviously, without any reasons at all).
You believe you have a virus, but no security suite can pinpoint it. Although you may just be oversensitive, chances are high that there is some kind of malware sitting somewhere on your hard drive, undetected by your comprehensive antivirus suites.
Programs take ages to launch, especially those that consume substantial amounts of system resources, for example Office suite, Internet browsers, or video and sound editing software. And if you don't leave an application just loading for a moment after it shows its window, it crashes.
You basically feel that you should buy a new computer. Such a thought is OK, I mean, who wants to deal with faulty hardware? But it may be just a matter of a few hours spent backing up your data (to USB sticks, external hard drives, a cloud service), formatting your hard driving, maybe wiping it down with another application to get rid of any residing data, and then reinstalling your Windows and putting all the important files back again.
Of course, if your PC still remembers dinosaurs it may be the time to purchase something new and shiny, but if it is just a two-years-old laptop that has stopped to meet your requirements you may want to rescue it before giving away your wallet. If you think that cleaning the PC and reinstalling the system is beyond your knowledge - hire a professionalist. It will still cost you far less then buying a new machine.