<product key>

ok so.. i got a new laptop from a boot sale. got it home all fine works ect but the girl who i got it from left her password on, so my brother-in-law had recently done me a windows xp disk (was ment for my pc, ) anyways i toke a look on the net and the only way it tole me to get rid of the password was to install windows so i did i wiped all her stuff of and windows, that went fine put then it asked me for the product key, well i thought it ment on the laptop so i entered that didn't now. so my brother in law done another disk and every disk we use it says invalid product key all tho the code is the way they gave him with the disk. Well he got me up and running but now its asking or it and i only have 28 day to finger this out, ANY HELP PLZZ???