Ah!!! it is a simple trojan....

right click on the start button, click on explore and start the explorer...
Click on the tools menu => Folder options
Select the tab "View"
Scroll down to "Hidden files and folder"
under that option click on the "show hidden files and folders option"
click apply
go to c:
you will now see that autorun.inf file.
open with notepad
you will see it starting a file (for example xyz.cmd) or making few registry entries...
locate the file xyz.cmd and delete it.
check the registry entries that it has made
finally delete the autorun.inf

simple...