sudden low performance in games and videos

[ovi200]

hi, last night i played bf3 and a bit euro truck sim 2 without any problems, today when i tried to play my fps was 10-15 fps also when watching videos on youtube or twitch in 1080p its stuttering and sound is buzzing a little . I made a full system scan with malwarebytes and avast which found no threats, i aslo updated windows and video drivers, reduce video settings to the lowest, shut down all unecessay programs with no change. temps stay between 30-35 idle and 60-65 celsius in games.
My pc:
Mb: asus P5B;
Cpu: intel core 2 duo E8500 @3.16 Ghz;
Ram: 4 gb ddr2;
Gpu: nvidia geforce 9800gt 1gb ddr3;
Psu: delux atx 500w;
Hdd: seagate 320 gb SATA II;
Case: ATX (http://www.lambda-tek.com/S-809-OEM-.../imgB94258.jpg).
OS: windows 7 64 biti
P.S.
-i made no changes or updates last night before i shut down the pc.
-i usually play bf3 at 30-40 fps (over 1000h)
-2-3 weeks ago when i instaled malwarebytes found 13 threats which i quarantined.
pls help :S

 

[hemantgta]

try reinstalling the game and install the latest patch and update ur gpu

 

[ovi200]

try reinstalling the game and install the latest patch and update ur gpu

hi,thx for reply, i just tried your solution and unfortunately it didn't work.
if there any information you require pls tell me, i want to fix this issue as soon as possible.

 

[hemantgta]

does this problem occured in other games .

 

[ovi200]

does this problem occured in other games .

yes, bf3, euro truck sim 2 and napoleon tw the only ones that i have.
Edit: a friend told me it could me my hdd or psu, but what i can't understand i how one i can play watch videos etc and next day performance drops with no changes to the system. i dont know when ill have the money to replace the psu or hdd or if its even worth it. i was thinking about reinstaling windows, what is ur opinion?
UPDATE: today after i started the pc i tried to play bf3 and it worked normally same for the other games, now im lost so one day it doesnt work next it works just like that?
i await ur oponions.