Files that are "lost" on a computer are usually still there, it is just the pointer to them that is lost.
If the pointer is lost then Windows cannot find them, but they may still be there.
There are various utilities that will hunt your hard disk for lost files. I have never used one so cannot say what one is best.
I went to download.com and searched for "recover lost files" and this list was returned.
http://www.download.com/3120-20_4-0.html?tg=dl -20&qt=recover%20lost%20files&tag=srch
You will have to see what one looks the best.
You could search download.com for other words like "hard disk recovery" and so on.
While doing this
DO NOT FORMAT YOUR HARD DISK.
DO NOT MAKE LOTS OF CHANGES TO YOUR HARD DISK. Lost files may be overwritten if the space is needed for new data.