You need to Defrag, Defrag, Defrag and then Defrag yet again. As the OS messes up the HDD the system will slow down and one instance of running Defrag will not be sufficient to cure the mess that is on the HDD.
However if it gets suddenly slow and remains that way you need to open the Device Manager and see what is happening . Any software can hog the CPU Clock Cycles slowing down the system dramatically things like AV Applications or Indexing Services are 2 of the more common things that can cause a system to slow down. So press the [b]CTRL, ALT and Delete Keys[/b] together to bring up the Task Manager and Click on the Performance Tab. Look at the CPU & Swap File usage which are displayed as a Graph. If one or either is high consistently left click on the Processes Tab and then on the CPU Heading 3 times to bring what is using the CPU to the top. A application like Process Quick Link Viewer 2 will allow you to see what is actually using what
http://www.processlibrary.com/quicklink/
This will rule out normal system processes as a possible problem though if you have Picked up an Infection it will probably not be so easy to find out and you'll need to restart your System in [b]Safe Mode[/b] by pressing the [b]F8 Key[/b] after the POST Screen goes out but before the Windows Splash Screen with the moving blue bar appears. When you get the White on Black Screen using the arrow keys highlight [b]Safe Mode[/b] and press Enter to start in Safe Mode then run your AV Scanner and scan the system.
You may be required to install additional Software to scan for Malware which are not Viruses depending on what you are using as an AV Product so start off with the Must Have Malware Scanners Spy Bot S&D
http://www.safer-networking.org/en/download/index.html
and Malware Bytes
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html?tag=mncol&cdlPid=10878968
and after these are installed and updated you can scan the system with them in [b]Safe Mode.[/b] Though as the time appears about right it could just be a Indexing Service that as installed with the last lot of M$ Updates and the system needs time to fully index the HDD before it starts to run as fast again.
Col