Recover files deleted from Windows 7 with no backup

I accidentally deleted important files, for which I do not have a backup. My thesis files worth four years of sweat is gone and when I tried to go back to a "previous versions" I get the message:

There are no previous versions available

How can I recover them from a Windows 7 machine?

First: STOP USING THE HARD DRIVE WHERE YOU DELETED THE FILE and do all recovery operations from a Live-CD. This will avoid other applications from overwriting whatever remains of your file.

You can try PhotoRec. Most of the time it should recover at least something, but if the old data has already been overwritten, you're out of luck.

You could try a few hard drive recovery tools, such as this one here. I haven't used them sorry so I can't let you know which one I'd recommend.

Also check out some of the questions asked right here on Super User such as

https://superuser.com/questions/128786/need-decent-undelete-utility-for-windows