To Replace Software:
See KB326246
"How to replace Microsoft software or hardware, order service packs and product upgrades, and replace product manuals"
http://support.microsoft.com/default.aspx/kb/326246To Replace Product Key:
See KB811224 "How to identify, locate, and replace a product key" http://support.microsoft.com/kb/811224
or
See KB824433 "Lost or misplaced product key (certificate of authenticity) for Windows" http://support.microsoft.com/kb/824433
(this doc is specific to Windows 95, 98, Millennium and Windows 2000, but should also work for XP).
Thank you,
Darin MS