need pc help! invalid version of windows xp

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recently my sons computer crashed and we couldn't get it to start again. since money is a little tight we were looking for someone to fix it for cheap. one of the guys my husband worked with said he could do it. he took it home got it going and gave it back to us. a few days later when I tried to download the driver for the cd drive that was no longer working I couldn't because I got a message saying our version of windows could not be validated. I checked and sure enough we were running a pirated version! told my husband to let the guy know I needed that fixed (I have a legitimate version of windows xp that I bought, i don't know why he didn't ask)and he tells me the guy got fired and he doesn't know how to contact him. I've been online trying to resolve this and the answer is to install a legitimate version of windows. easy enough since I have it right? wrong. because the cd drive won't work without downloading the driver and I can't download the driver until i've installed the legit version of windows. I can download windows online, but then I have to pay for it which I don't want to do since I already own the disc. soooo....any computer whizzes out there have any advice on what I can do?