As far as I understand if you own a retail copy of Windows you can request a replacement disc directly from Microsoft, if so contact them via their website or phone customer services.
If you have an OEM license for Windows Microsoft will not give you a new CD, and it is up to the discretion of the original PC vender who supplied you with a copy in the first place. Normally they won't as OEM are cheap for a reason but it's worth asking
Get yourself a copy of Windows 7 home premium its cheap enough or get the upgrade version and you can upgrade you existing Xp installation.