Problems:
Your pc speed is getting slower.
Cpu performance is bad always laggy.
Have tried monitor blue screen.
Solution:
Well there are may kinds of solution for your pc problems.
It will be best changing cpu drives or formating your pc.
But if laggy is your only problem this is the answer for that.
*Buy a THERMAL PASTE also called as thermal gel,thermal grease,thermal compound or heat sink paste.You can get it any where in your places like groceries,shops for pc or malls.
*Just apply it to your processor like the picture above.