L,
You have my sympathy, it is usually just one minor thing that causes all your problems. Finding it is the nightmare!

One option is to install a new hard drive in the computer and put XP onto that. You might even have one lying around in an old external USB hard drive in back of a cupboard. It's also worth asking around if anyone has a working XP era PC they'd let you have cheap. I did that on my local 'Buy, Sell, Swop' Facebook page a while back and had 2 by tea-time. A hybrid of those is what runs my router now.

Good luck

Kit