how can we delete a file.?

Clarifications

The short answer is : you can't

On modern operating system when a program opens a file it tells the OS how it intend the file to be shared with other programs; On your computer at least one program has opened the autorun.inf file while requesting other programs to be prevented to delete this file.
First you will have to find which program(s) hold the file and then you will have to terminate this/those program(s). As far as I understand from tags associated with your question you are using a Windows OS. "Process Explorer" a free tool from SysInternals may help you to do so. The company has been acquired by Microsoft and their tools are now available from the microsoft.com site.

Caution : If a program opened the file and prevented deletion there may be some very good reasons for that. Deleting the file may prevent the program to run anymore.

deleting system files anywho....here's a little info on what you're trying to delete http://autorun.moonvalley.com/autoruninf.htm

If the file resides on read-only media - you can't delete it. So if your autorun.inf is on a cd, you can't delete it.
If the file is constantly in use however, there is an option. It's called unlocker.
http://download.cnet.com/Unlocker/3000-2248_4-10493998.html
Warning, if you screw up your OS with this program, don't say I didn't warn ya :)

For some people it the only way to learn. My question is why do you want to delete autorun.inf it can be disable if your not skilled deleting is just bad if you just don't want a cd to run hold down shift when you put it in.