:: Do the following ::
1) Disable autorun/autoplay function of your pen drive.
2) Now plugin your pen drive and open any folder of your computer.
(Do not open any folder from ur pen drive.)
3) Now goto tools then select Folder Option
In that box mark the Show Hidden Files & Folders option.
Also Uncheck Hide Extension for known file types & Hide Protected OS files.
Now click apply and ok buttons and close that folder.
4) Nome come to desktop.
On the Desktop, click on windows Start button and select Search for files & folders.
When the search dialog box appears on the screen, in that click on All Files & Folders, now click on More Advanced Options then select search Hidden files & folders.
Now go above & in Look In option, select your pendrive letter (For example E: Or G:)
and hit Enter.
5) Now if you see any unknown .Exe files, simply delete them all.