& if you played with Hardwares, like removing/replacing something, then make sure the HDD cables are inserted firmly, RAM is/are seated firmly in their slots.
coz i did that

, to clean the mobo, CPU fan(heat sink), i had to remove everything, but after i assembled them back again, my PC would get randomly Slow. the mouse pointer would flicker heavily when moved.
the problem was- the HDD SATA cable was not fit firmly. (got loose when i fitted Floppy cables

)