Computer suddenly become very slow when playing games

What are your specs?

Intel(R) Core(TM) I7-720QM Mobile Processor (1.6GHz, turbo up to 2.8GHz, 6MB L3 Cache)
4GB (2GBx2) 1333MHz DDR3 SDRAM
500GB 7200RPM SATA Hard Drive
1GB GDDR3 NVIDIA(R) GeForce(R) GTX 260M

It is possibly overheating try running CPUID HWMonitor to check the temps.

Temperatures is 45 C. Is that too hot?

Is that during slowdown?

I think so. I just do some browsing and downloading

Fire up the Task Manager and look at the Performance tab...keep it open. Play your games, in windowed mode if possible or on another monitor. When the lag happens check the Performance screen to see if RAM is being maxed out or if the CPU is at 100% etc. Also keep HWMonitor open and see what the temps are at the same time...'I think so' doesn't really help us diagnose your problem. Help us help you.  

Also check that your anti-virus isn't kicking in at the same time, might just be running a scan. It would be an idea to run a defrag, CCleaner sweep and a virus/malware etc scan too.

I just update my graphic driver... and its now back to normal! But I still don't know why this problem happened. Anyway, its all done now, thanks for the fast response