A new operating system installation may have overwritten
old file system information and may have included a hard drive reformatting.
If the hard drive was formatted and installed,
the new OS may actually occupy the same general area as old
operating system. This can actually benefit the
recovery because the actual data may not have been completely overwritten
by the new operating system.
With Macs, data will be overwritten by choosing
"New Installation" instead of "Archive and Install" when installing
a new version of OS X.