Hello Mazeedullah Niamat,
This MS KB article covers How to Activate Windows XP:
http://support.microsoft.com/kb/307890
To manually start the Product Activation wizard:
Click Start, click Run, type oobe/msoobe /a, and then click OK.
Note There is a space between msoobe and /a.
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp