What it sometimes comes down to is you take an educated guess and fix it if you guess wrong. Or you can post to a bunch of user forums and hope that you get the correct answer from one of them.
Could you try to explain again what your computer is doing or not doing that has become a problem? Is it slow internet browsing? Slow startup? Locking up frequently? You mentioned something new on your desktop? Did you google that thing? Any programs that you downloaded from the internet can be removed, since you can easily install them again if you miss them. Again, there's not a whole lot (relatively speaking) that you need to perform general tasks like browsing the internet or email messaging, creating documents, pictures, printing, etc.
All the "extra" programs that provide widgets, bells and whistles usually communicate with their creators and send information about how you use your computer. That slows things down. Suspect EVERY add-on toolbar.