Why is my computer running so slow?

I was looking to buy a new computer because for the last 6 months the computer is working so slow. I defrag it about once every two weeks, i delete old programs. but my computer still wont work. My Computer is a Dell that is rather older. about last year it sayed that the updates stopped. so i clicked to update it but it didnt respond and had to exit out of the program. idk what is wrong with it. i even downloaded the Avast anti-virus system. Someone said to me to go into safe mode and set my computer to an earlier date. i tried that too. i dont know y its working so slow. i have tried everything that i know. i know its not my network. somebody help me please...

Why is my computer running so slow?norton internet security

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Why is my computer running so slow?spyware remove

If the other answers don't work, try this site.
Have you deleted your start up programs?

Go to start menu and type MSCONFIG in the RUN tab.

Go to startup tab.. it will list all the programs that startup when the computer starts up and continue to stay running..