The files you deleted from your disk can be easily restored into a readable form. Expensive equipment or special knowledge are often not necessary.
Good news for those who have deleted an important file by mistake, bad news for our privacy.
Operation system usually deletes files by labeling them as free space. They are easily restorable by means of small applications which are available on the Internet in large numbers. Defence is simple: place a new file on the same disk spot which will cover the original one and prevents its restoring. How do you tell the system that it should place a file to a particular spot?
Nevertheless, the data can be retrieved even after being rewritten with another file. This is caused by the physical way of storing data to the disk. Todays security standards recommend to rewrite the file on the disk up to 35 times. These standards do not only set the number of rewriting but also what patterns of binary data should be used for rewriting.
Secure removal of sensitive data should be carried out by each user in his own interest. Fortunately, there are security tools which make unrestorable files removal easier and automatic. Do not forget to safely delete the disk in case you are selling it or throwing it away!