There are a lot of possible reasons your computer could be running so slowly, one possibility is that it is infected with a virus or something similar. Another possibility is your hard drive could either be too full, or may have physical problems.
I don’t know how long it’s been acting like this, or really enough information to diagnose the problem, but I would have it looked at.
I would also strongly recommend that you back your files up, since chances are you never have (the vast majority of people never do, which is a huge mistake). Do this before defragging your hard drive, if you try that, since if the drive is bad (as opposed to just infected with something) you could kill the drive in the process of defragging.
This is not to say that defragging a drive is normally damaging, just that *if* the hard drive is failing (and of all the parts in the computer, it is the most likely to go bad in my 13+ years of experience as a computer tech) the extra work of defragging can be the last straw. It’s kind of like someone with a heart condition dying while shoveling snow or exerting themselves — they may seem OK in normal circumstances, but the sudden extra heavy work causes their heart to fail.
Not a pleasant image, I know, but it gets the point across.
Anyway, back up your files, then scan the computer and see what happens. Or better yet, get a professional to look at it. Good luck with it.