Lag problems while playing games

Hi, recently my laptop has been lagging every few minutes when i play a game. i press ctrl+alt+del to check which programs were active during the lag and notice that "explorer.exe" and "System" are taking up almost all of the processing power, when the game is running it usually takes up 80-90 of the processing power and goes down to 0-20 when lagging. What could be wrong?

(im from norway and using the norwegian version of windows and dont speak english perfectly and dont know much about computers so if i havent made much sense, just ask)

*btw ive had this laptop for 2 years and didnt have this problem before. and also my laptops motherboard was replaced more than a month ago cos i sent it out cos the fan wasnt working very well. I wasnt having this problem til about 3 weeks after i got my computer back. =(
 
How old exactly is the games your playing? It could be a bad processor or a dying processor
 
i never ugraded it. but isnt the processor part of the motherboard? i dont know, but thats what the service ppl replaced when i sent it out for repairs.
 
i sent an email to the service guys and they said it might be windows thats causing the problem. im not sure if i should reinstall it =(
 
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