PC throttles whenever enemy on screen on any game

Anygame, be it CSGO or Fortnite, its all going great at 30-60fps, Untill an enemy comes on screen. the moment theres an enemy, the pc Throttles and fps drops to 5-7 and its all soo laggy for a solid 35 to 55 secs and i getclapped on immediately. it was normal until 2 weeks ago, then it started completely suddenly. i used to play fornite 2 hrs a day. the suddenly at arounf the 1 hr mark..game used to crash.. every day for a week... the this lag started. please help.

Go in to the graphics settings of each game and lower the effects qualities. Shadow quality and Anti Aliasing are usually good ones to reduce or switch off with very little effect on visuals but will freeing up of resources to help the frame rate.
What controls there are available will depend on the particular game, descriptions might vary a bit.

You may also have an overheating problem, so install a temperature monitoring application to check, obviously just prior to the crashing.

What Helmut & CountMike said.

Also, I see your specs say you have no anti-malware. Do a malware scan at one of the online vendors, in case you've been infected.

Your computer doesn't look too slow for the games you are playing.

If you have Nvidia go to the Nvidia Control Panel Manage 3D settings, select the game and then choose Prefer Maximum Performance for the Power Management Mode and hit apply.

Make sure you also turn off GameBarDVR. Here's a good tutorial on how to turn off GameDVR.
Perhaps this may be the reason for your fps drop. I hope this solves your fps drops. If it doesn't then your problem lies elsewhere. If you didn't have fps issues with a previous operating system then GameBarDVR is probably the issue in Windows 10 that causes fps drops. If you had issues in previous OS then Windows 10 won't be the blame here.