When running some heavy applications like games or virtual box for some time.. the cpu usage is normal for some 15 mins and then suddently cpu usage increases. even after i quit the heavy apps and when i start some other apps, the cpu usage of the new opened application is also very high.. this continues until i reboot the system. There is no single particular process occupy more cpu. All the processes cpu usage is little high than normal.. Any solutions?
Is it possible that Intel SpeedStep or AMD's equivalent system, is throttling your CPU usage for power management. Check what power setting your computer is set to; in XP, the "portable" and the "max battery" settings will step the processor down to its lower speeds. You can use CPU-Z to see what clock speed the processor is actually running at. If your processor is running at 1.0 GHz instead of 2.6 GHz, for example, then suddenly all of the processes will be competing for a lot less power.
Task Manager is only part of the picture (CPU) -- it's possible disk I/O or heavy memory usage is causing the CPU usage to spike.
You might try running Sysinternals Process Explorer to get more visibility into memory and disk I/O patterns when this occurs..
Or as Bobby commented, it might be a heat problem. You should install a sensor and then examine the temperature of your cpu.
Example: link text
In linux you can control the frequency of the system in /proc, how you're doing it in Windows, I don't know, maybe someone other knows something