Do you worry about privacy or do you maintain some personal files on the office system or at Friends pc, then you must lock that folder. I know many software's that most people use to lock their Personal files. But this trick is the safest and locks files or folders without use of any software's.  

           Suppose you want to lock the folder movies in d: which has the path D:\movies.

In the same drive create a text file and type

ren movies movies.{21EC2020-3AEA-1069-A2DD-08002B30309D}

Now save this text file as loc.bat

Create another text file and type in it

ren movies.{21EC2020-3AEA-1069-A2DD-08002B30309D} movies

Now save this text file as key.bat
     Now you can see 2 batch files loc and key. Press loc and the folder movies will change to control panel and you cannot view its contents. Press key and you will get back your original folder.
try it out!!!!!!!

Note: It is safe as in case if you loose the key file, you can create it again but the folder name must be correct.