|If you have XP on the subject computer, XP does not have Dos but it has a black screen cmd mode .|
Start - Run - type: cmd
or you can press F8 repeatedly while booting and choose to boot into Command mode.
Typing systeminfo works there - expand the window, use the scroll bar at right to go upwards - the mhz is at the end of the Processor(s) line.
In Windows (95 and up)
Start - Run - type: msinfo32 , click OK or press Enter - the info you want is on the first screen you see at the right -
RIGHT click on My Computer - choose Properties - the processor mhz is shown on the first screen you see, on the right.
"after i boot my laptop. again after 1 or 2 day the laptop ask 2 reboot again"
There's not necessarily anything wrong with that. If you get a message asking you to do that, or asking for permission to do that, there's probably nothing wrong. Some things want you to restart Windows - e.g. such as when your anti-virus software updates it's program, it may want you to restart Windows to load the changes.