Question : How to know which task or service the taskmgr.exe is running

Hi!

Sometimes, when otherwise idle, the Windows machine begins to run just by itself, and on the Task Manager you can see taskmgr.exe running highest on some 10-20% maybe some 5-10 mins time.

How to know what task or service is then run there?

Answer : How to know which task or service the taskmgr.exe is running

Order it by CPU usage or by memory. You will see which processes are eating your resources.
You may try to use "process explorer" from Microsoft, it is more detailed and powerful that Task Manager.

http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx