You can use a program like FreeUndelete...
http://www.tucows.com/preview/502837
When you delete files they are not instantly gone forever... yet. What happens is that the "identifier" in those files have been deleted. The files themselves still remains intact on your hard drive until something is written to the hard drive, like downloading and saving a file.
For example, if your current hard drive has 10GB of free space and you delete 1GB of files, you now have 11GB of free space. But those 1GB of files are still on the hard drive; they are considered as "free space" so the next time you save something to the hard drive it is possible that what you save will overwrite some of the clusters that the 1GB of files you deleted use to...