You would think that Windows would encrypt or even trash away your Product Key after install, but no, it is embedded right into a text file!!!!
The actual key used during installation is located at
%windir%system32 . But if i cant find it, just use windows search companion search for >>> OOBEINFO.INI
Open the OOBEINFO.INI file and the product key that was used during installation in at the bottom/ top (for me only la).
I did this cos i bought laptop from school, and i burn 3, yes 3 bootable windows xp discs in case kena virus.
FYI: this works for xp only, vista? no way