One way to get your PC running like new is to backup all your important data, then do a complete clean installation of Windows and all your programs.  That method will do the job, but its a lengthy process.

The alternative is to use specialist software to fix problems and generally maintain your computer so that it always runs like new.

One of the most popular programs is System Mechanic which uses over 50 built in tools to seek out problems that slow down computers, then make sure everthing keeps running as it should.

Verion 10.8 has just been released which is compatible with the beta version of Windows 8 and you only need to buy a single copy to use it on all the PCs in your house.  Get more information on Sytem Mechanic at http://www.iolo.com/system-mechanic/standard/