Incorrect Serail Number

A bit of history b4 i get to the point..
bought a p,c with preinstalled win xp home on it no sp2 that i needed so after finding out that the copy on here was banned i new i wouldnt be able to d/l sp2 thankfully i had my own copy of xp home so i completely formatted (fdisk) and installed xp home only problem is.. it told me serial was incorrect i checked double checked tried several times and eventualy it let me on with a 30 day authentication time limit i got round that and guess i had put it in wrong however since that ive bought xp pro full disc over the counter at a reprable outlet but again it says cd key incorrect ive taken it back and got a change but gain second copy says same erro ive tried it on an old p,c so know both disc and key are working but my p,c wont have it.
Please help as im running out of ideas and im devastated on spending big bucks on a disc i want but carnt use

 

tried calling microsoft today explaining the above. they gave me a new number ..that doesnt work either all they could suggest is i have an error on my machine possible things they recomended to check was 1. bios settings.. ive checked every last entry against other p,c they are all the same..
2. a partition could possibly be stopping a new xp version.. there is no partition ive even tried changeing the hard drive with no luck

seems im stuck again