You need a 'temporal time-shift' PCMCIA card - should be able to get one from PC World. I think HGWells Computers and DeLorean both make them (I think DeLorean's is called the "Back to the Future" card) - careful how you use it though as you could find yourself back in time before the invention of the motorcycle!
Seriously though mate - you are b*ggered. One slim hope is that the when you overwrote the file the data was actually saved to a different physical place on the hard disk and the original file (or some of it) is still there. In that could maybe use a low-level utility by Norton or similar to access the disk-drive directly. Not sure what Norton is calling this utility at the moment - they seem to change the names of their programmes every year or so. Last time I lookedit was called Norton Disk Doctor I think. Best of luck!