If your system was "completely formatted", where did the 40GB of data come from? If the 40GB of data was actually "Backed Up", I can see how the bill could add up fast, but 40GB of Data is HUGE!!! That would be equal to over 10,000 really high resolution photographs,
or 10,000 songs. If you have that size of backups you should have it backed up anyway. But I suspect you're being scammed. First of all, a brand spanking new copy of Windows XP runs around $95-100, not $180. And if they destroyed your operating system, charging you for a new one is ridiculous. In our shop, we charge $169 to reformat and reinstall an operating system. We add $30 for unlimited data backup for a total of $199. We do this about 15 times a week.
But the nuts and bolts of your question is more problematic. As pointed out earlier, your laptop may not run on Windows XP due to the driver issue. Numerous laptops have been configured to run on Vista only. So you could be stuck with Vista. But one way to check it out would be to simply load XP with any valid KEY just to test it out. With just a little looking around, you should be able to get your hands on a XP Disk and KEY. You won't be able to use them long term, but they would work for testing. Install XP and see if you can get the hardware to work. You may need to hunt to find the initial drivers using a different computer, but once you get online you can try a paid site like driveragent to scan your laptop and locate your drivers. If everything works, you can use XP, if not you'll be stuck with Vista. By the way, if you're stuck with Vista, you have a valid licence KEY on the bottom of your computer. You should not have to buy it again under any circumstances!!! You may, however, have to pay to get it loaded on your hard drive, and you may wish to invest in a disk set (usually about $35) from the manufacturer.