Threre are several methods you can try to fix your issue but they all require an XP CD with recovery console to repair or reinstall XP.
As far as obtaining a copy of XP that's your choice you can purchase a copy or perhaps a friend can "err", "cough", "cough" ,"lend" you a copy.
This link has several solutions for your issue but honestly in most case's you will just end up having to reinstall windows (meaning you will lose your data). If you have a second PC you can proabaly remove your CarPC hard drive and copy your data but you will have to reinstall all your programs.
In the future you may want to consider using an imaging software such as paragon drive backup for making backuups of you OS once configured.