If its a computer built by a vendor, then it has a sticker on it.
Call the vendor, and get a replacement cd.
Dell charges 10 bucks. HP i think is similar.
They usually throw in the driver cd, so you can reinstall.
If someone built the computer with a bootleg copy then suck it up and buy an oem copy on ebay for like 20 bucks or whatever.
Or here is the best option, if its just a bullshit computer that she surfs the internet and checks email, then put linux on it and call it done.