Hi,
The first thing to do is stop any more writes happening to the disk. Power off the system so that no further data will be lost. Make an image of the disk and work on a copy so you don't risk losing any more data.

There are tools to recover deleted files. Check out The Sleuth Kit (TSK) and autopsy for a free way of examining file systems. A reinstall of the OS probably means that you've formatted the system volume, so this won't be a simple case of finding deleted files in the current file system. A data carving tool like foremost might be your best bet in recovering files.

Can you let us know the OS, file system type (before and after installation) and the type of files you are trying to recover?

Regards,
Jimbob

Foremost is an awesome tool.  I really think its data carving ability beats most of the commercial tools.   

One thing you can try is a couple of inexpensive tools that do a search for lost partitions.   You may be lucky and be able to recreate the lost partition.  TestDisk is one such tool.   I have used it before with much success.   If you have access to EnCase or X-Ways Forensics, they both have some nice tools for recovering data.