Hello izoran,
Please let us know what version and edition of Windows is described on the Microsoft Certificate of Authenticity you have.
The CoA for an OEM license for XP is located on the computer case (the bottom if a laptop) while the CoA for a retail license for XP is located on the boxtop of the pretty retail box that the CD came in.
DO NOT post the product key that may be printed on the CoA.
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp