You can start over with the old disk and it's product ID code. But, if you can download the full SP file from MS which is about 320 megabytes, you can use "autostreamer" (look it up on Google) to make you an SP3 disk. You just need a burner and a program such as Nero that can burn iso disks. This will save you lots of update time but there will still be plenty more after that. Good luck.
I am currently running a computer with XP Home SP3 installed and want to format the hard drive and 'start again' as the computer is extremely slow.
However, I only have an XP Home SP1 CD. Will I be able to install from scratch XP Home SP1 on that computer using an SP3 licence key? Or will I need to get an SP3 CD in order for it to work? If that is the case where can I get one?