How Can I Get My Window Professional Serial Number

I don't have my window professional serial number any more after using my system for along time, so someday i decided to change my system by removing the motherboard, drive etc to a new frame.
then i after everything i put in the power, while it works BUT IT IS  REQUISTING FOR THE SERIAL NUMBER What can i do,

you can  download 'keyfinder thing' or 'MS Key ViewerPlus'

Hey! try this key:
k2kb2-bdbgv-kp686-d8t7x-hdmq8
win. pro.  05

@Topic

You mentioned changing the system, how do you mean? Did you change the system as in "got a different motherboard?" or "just swapped the existing configuration into a  new casing?"

If you swapped the motherboard, Windows would not only ask a key, it would also require reactivation over the internet. This happens most times when major reconfiguration has occurred to the system since the last activation/installation.

If its just the case you swapped, then I really am surprised that its requesting any key again.
Does the system boot up to the desktop? or the "it works" you are referring to simply means it powers up?

Thats rather strange then! I've never seen where a system just asks you to enter a serial number. I guess what it requested was for you to "activate your copy of windows". Then you opted for the "activate over the phone" option.

Well, if that is the case, if you copy of windows is genuine(or an OEM copy), then get your system online and activate directly and forget about getting the serial for now. If the copy is not genuine, then you would have to get a valid key(buy from microsoft or leech off someone else) to get over this.

hi Maleeq thanks for that i thinks it would work,