actually, you can set the computer back to day one or another day using system restore. You can do a repair install of XP without reformatting the drive, and without erasing your personal files. Those two choices are easier than doing a complete re-install. But you may still need the motherboard drivers disk or download the drivers from the manufacturer website. And you will need your printer install disk, etc...
It's really cheaper to buy the XP SP3 pro disk, like on Amazon or other vendor, and better to have the disk in case problems happen later. The disk will cost about $60.
The download of XP SP3 ISO will cost quite a bit more, and then you will not have an original disk to repair with.
The final avenue (this works) is to load and reformat, fresh install SP2 from the disk you have now. During the install widows will ask you to connect and find updated versions. If you have internet connected, it will download the latest versions. At the end, you can use your old key number, or select "I don't have a key number" and microsoft will allow you to pay them over the internet (it costs more). It's a good choice to connect while installing and use the newer updated version.
Again, you can go to the microsoft website and download a full SP3 but they charge you a lot higher price than buying a disk from Amazon.