you can TRY to use software to deep scan the hard drive for recoverable files: http://www.piriform.com/recuva but don't count on recovering every single file. Sometimes the files that are recovered are still corrupted and may cause errors when opening them or using them.
95% of the time, you are NOT going to recover the lost data. If you ever have something like that happen, do not ever install, create, or do ANYTHING that will cause the disk to possibly overwrite critical files of yours.