In case anyone cares how this all works: When you delete a file, you actually don't remove it from your drive. There's a big table that says basically "A file called PICTURE.JPG is right here *point* on the hard drive, two inches from the left on the 4th platter, and it's 1.2MB big. No other file can go here, this space is occupied." When you delete PICTURE.JPG, the entry for that file is removed from the table, but nothing else happens. Two inches from the left on the 4th platter, the guts of PICTURE.JPG are still there, but Windows pretends there's nothing there at all. 15 minutes later when you need to make a new file, Windows will say "Hmm. I need some free space to stick this file. Well, two inches from the left on the 4th platter is empty according to my list, so let's stick it there!" and only at that point is PICTURE.JPG gone forever.

If the drive was so full that you were getting critical low space messages, then it's very likely too late for many / most of the files. ANY new file that was created since the pictures were deleted have to have overwritten free space that was previously a picture, because there is little or no other free space to use. Even if you didn't make a new file, as Yams said, Windows swaps crap to disk all the time, compulsively, and each time it does, more picture data is gone.

If you ever delete a file you think you need to undelete, the best thing to do is immediately stop doing everything on the computer and turn it off. Then boot from a CD or floppy and try to recover it from there. However in this case it's quite possibly too late for that. Everyone learns the need for file backups approximately ten seconds after the first time they lose all their files and realize they have no backups. It happens to us all.