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The easiest way to speed up your computer is by going to Start menu > Run... and type MSCONFIG (or press Windows key + R to get the Run box to come up and type MSCONFIG) then press [ENTER]
Then go to Startup tab and click "Disable All" then click OK. Do note that you can always access any of those programs by clicking the icon on your desktop or by going to Start menu --> All Programs. It just prevents them from eating up memory while you’re not even using them down by your clock.
It's also good to defragment your hard drive to keep file access speeds faster: http://www.auslogics.com/en/software/disk-defrag/download/
Once downloaded and installed, simply click "Defrag" to have the program defragment your computer and put files in a more organized fashion on your hard drive. This can take anywhere from 15 minutes to 2 hours and should be performed at least once every few months.
If it still freezes up after taking these steps to basically have a 'clean boot' I would suggest removing any anti-virus/security software you have running and see if the freezing stops because it's possible that the security software you have is causing that... You should only have one realtime antivirus/anti-malware program running otherwise that can create issues, as well.
If it still freezes up, and/or the system seems fine for a longer period of time if left turned off longer then you're looking at a possible failure of a cooling fan inside, either on the CPU or just the intake/power fan.
If the freezing is inconsistent, where the system will lock up without you even really doing anything, then it's possible your hard drive is failing, especially if you aren't really doing anything and it locks up. Easy way to check is by just randomly opening folders for a minute or two to access different locations on your hard drive.
If it does end up seeming hardware-related, then you are covered under your manufacturer's warranty and can most likely have someone come out for free. Thanks!