Just bought a laptop cheap as on trade me for extra battery was told that it was broken but fired it up and works but is asking me for a user name and password to log on to windows. can i over ride it or should i take battary and bin laptop

Keep the laptop, hope it got a working cd-rom/dvd-drive !(if not, take-out /replace the harddisk)
-win xp has to be removed from harddisk,
the hd must be formatted so you got a blank hd to set up your new system

you'll need :
- antoher PC with internet connection and cd/dvd-burner
- a win98 installation cd or bootable savecopy of it,
- a new win-xp install cd plus cd-key
- a program called PC Wizard(download)
and a bit of time...(one night or two:-)

follow instructions:

if you can enter bios (press F1 or F2 or Del)
during booting), youre lucky...

enter bios while booting, in the menu look for boot otions or devices, change order of boot devices, so that cd/dvd-drive is 1st, then save and exit (F10)
insert win98-CD, boot from cd, try to setup win98, you'll be asked to erase/ format all data/installations/partitions
on the harddisk (drive c:,probably), do so, format the drive using the FAT32-file system,
then end installation without fully setting up win 98.
now you should have a blank disk witout any operation system on it.

now put in win xp cd, go through the whole installation process
install and run pc wizard(it'll tell you all hardware devices of the laptop)
get the proper drivers for the notebook for win-xp from the net(google might be helpful)
install them.
happy computing!

if all this did not work, remove the harddisk,(you'll have to unscrew a lot of the laptop)
replace it by a new s-ata ide-drive()and..
erh.. come again then, this is a long process as well