Finally, when I was able to delete that directory (only about 15 Mb in size), suddenly Explorer showed 23 Gb free. Gasp!
Since you already deleted the folder, I will not be able to confirm it but I think you were hit by a trojan. I cannot recollect the name. The antivirus usually is not able to detect it. The best way to check for it's presense is

Go to that folder and go to Tools, Folder Options, and click the View tab. Select/Check the option to Show hidden files and folders. Click Ok and close that folder.

Open that folder once again and go to Tools, Folder Options, and click the View tab. If the "Show hidden files and folders" is unchecked then that confirms the presence of the trojan....

BTW, could you check the above with any other folder for me?