My post probably is not going to be direct, step-by-step help to you because I am not allowed to post the utilities that I ordinarily use on this forum website. Generally.....if your computer is slowing visibly but is running smoothly and normally, usually this is caused by one or both of two things:
> You have too many loaders starting on boot. These software packages load into memory whenever you boot the computer and chances are that you do not know they are loading. Some of the usual suspects are:
** In the case of a retail unit that you purchased from a store/online: adware, spyware, trial-ware, and just plain garbageware. The usual are Symantec Norton antivirus trialware that has expired but is still loading on boot. Various McAfee trialware; Microsoft Office trialware; various free games and utilities, and so on. In addition, companies like HP, Dell, Acer and others have what I call "overlay" programs that "help" you configure settings in Windows that Microsoft has included perfectly good screens for doing the same thing. These load into memory and sit there doing nothing 90% of the time the unit is running. One for example is common: a "helper" to configure your internet settings. Microsoft has a perfectly good and easy-to-understand screen tool for that, and all they do is inject a screen that uses the same Windows configuration utility and supposedly "helps" you to do it easier. That is why I call them "overlays", they just "overlay" a screen that uses the same M/Soft config tools. Most load on boot and just sit there doing nothing.
** BHOs. This is a big one. I have taken as many as 12 Microsoft Internet Explorer entries loading on boot off of clients' computers. There is never a BHO that I found to be actually necessary, I nuke them all and afterwards the computer starts running noticeably faster.
** Tool bars: programs love to load you up with their custom Tool Bar. I take dozens off clients' units all the time, and the majority don't even know these tool bars (and BHOs) are loading.
** "Update Checkers": most programs love to have a loader automatically check for updates to their programs. After a few years you can have dozens all loading into memory and sitting there like fat pigs eating clock cycles off the CPU and doing absolutely nothing other than occasionally checking to see if there is a newer version of their program available. You can just as easily check for updates yourself manually. And....if you are like me....I have small utilities that run very well and I really don't care if a new version is published or not.
There are Spyware/Adware programs running: I suggest you run ADWcleaner and JRT (Junkware Removal Tool) available from BC. Let them remove anything they find. Both are available from this website. Do not use any of the others listed if you are not acquainted with them.
Autoruns: download Autoruns from BC; this powerful utility will list pretty much most of the things loading on boot. Be careful with this tool, study it first; you can damage your operating system. I suggest using the filter to not list Microsoft entries, that will help protect the OS. There are many tutorials and videos on using Autoruns.
HIjack This: this tool used to be the "go to" for virus removal, but is pretty much out-dated now. However....it lists certain places in the registry that can contain auto-loaders and can target things that load on boot, and sometimes can be very helpful removing tool bars or BHOs. If it will not remove something that you see should not be there, at least you now know that it is there and can take other action to remove it.
If cleaning out the system does not help, then you may have other problems. Jump over to the virus/malware removal section and have someone check over your computer, at least you can get that possibility covered. Hope I helped a bit.