First off, whenever something like this happens, immediately shut your computer down to prevent Windows or any other program overwriting the data. Get yourself a copy of one of the best freeware recovery software around, PC Inspector File Recovery
*www.pcinspector.de/file_recovery/uk/download.htm
If the data is on another drive than Windows, you can safely try installing the software in your computer, then running it to recovery your data on the other drive. However if its on the same drive as Windows, the best thing to do would be to unplug your hard drive and take it to a friend's place, plug it in there, install the software on his computer and then recover the data. PCIFR will tell you if the data integrity is poor, fair or good, and that will translate into whether you will be able to recover your data or not.