it has been downloaded by my laptop "rundll32".it is some sort of app,but i do not know what it is and what it do.it is 54kb the size and for a moment it have taken 98% of my cpu power.anybody knows what it is?i'm a bit worry. thanks in advance.
rundll32.exe is a Windows application that executes shared stuff from DLLs. It's totally legit, so no worries about malware, but if it regularly eats CPU like that while idle you may be looking at a poorly developed DLL that's eating more CPU than it should.
You can find out what DLL rundll32 is running by opening Task Manager, selecting the Processes tab, and making sure the Command Line column is visible. That column will show what argument rundll32 was given.
rundll32 should only appear in TM when it is actually handling a dll, and should only use CPU time as it handles; most of the time it should show zero CPU time while there. As an example, to get rundll32 to appear, rclick your taskbar clock and choose Adjust.
You could check its properties; the valid rundll32.exe is in system32. Further, if you delete it, it should be replaced by Windows File Protection from cache immediately.