Sounds like Hard Drive is going bad or even the cpu fan might be bad and not cooling down the CPU properly:
Ok lets start by doing this first if possible:
1. Hit ctrl \ alt \ del on your keyboard to bring up your task manager.
2. Select the processes tab and maximize the window.
3. Hit Ctrl \ Prtscn on your keyboard then open up Microsoft Paint and then hit control v on your keyboard.
4. Save file as a jpg or gif then paste in a response to us.
Also if you can download
5. Donwload a program called speedfan and open the program and when the noise starts check to see if any fans are slowing down !
This will tell us what is actually running in the backround for the most part and if your cpu is running hot to make your programs run slower!
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MiKeY
MCITP MCSE CompTia A+ Server+ Security+