Well, as far as the partitioning scheme goes, Sony has AFAIK always shipped there computers with a C: and D: drive, so that in the event the OS neeeds to be reinstalled (as with yours) One could do so, without having to format all of their personal files which "should" be stored on the D: drive. I am not sure how "recent" you bought it, but you may be able to go to Best Buy with your receipt, and laptop, and other documentation that you have, and explain that you never received the system recovery disks, but you DO have the product key. (The Windows sticker) If they can't help you, then contact Sony. I think that most (if not all) major manufacturers can provide people with copies of the media, for a charge. I don't know the prices, but I am sure it is significantly less than purchasing a new copy of Windows. In your case, you already have the license, now you just need the media. There is no way to reinstall XP without the disk. No "online" installations or anything. I don't think this is possible with Windows. Once you do obtain the Windows XP media, it would be a smart idea to copy all of your personal filesto the D: drive, if you don't already. Or, maybe you can copy them to DVD and then reformat all 120gb as on big C: drive during the Windows reinstallation. There is the option of doing a Windows recovery once you have the XP media, but I would personally recommend a full format.
good luck, and let us know what ends up working out for you!
Good luck, and post with what happens.