Don't format! It seems like as soon as anyone sees a problem, they like to format - I know, it gives you a nice feeling, but it can also be one huge mission. Also, even if you format the drive, the virus will just find it's way back on when you plug in your hard drive. I've seen lots of viri (?) like this before. It copies itself onto any emovable/network drive it sees, as well as secondary hard drives in your pc (sometimes all of these, sometimes just one). What you need to do is run a full system virus scan, including all drives (have your eternal plugged in for this). If anything shows up on the drive where your Windows installation is, view the currently running processes, and if you see the same exe file there, end the process and then find and delete the file.
If the virus scan doesn't show up anything, then you'll just have to look at the currently running processes, and find something that you're 100% sure isn't a Windows process, or one of your own running programs. If you find one (like temp.exe or something) then end the process and find the file (probably in the system32 folder) and delete it. Then go to run, and type in msconfig, and in the startup and services tabs, untick anything with the same name as the file you deleted. The run regedit, and search for the exe that you deleted, and delete anything you're sure is directly referencing it. Then enable viewing of all hidden files, extensions and system files, and on all of your drive, delete autorun.inf and (apart from on your system drive) delete any exe files that you never specifically put there (like host.exe or copy.exe).
Now remoe the external drive, and restart your computer. If it still happens when you plug in your drive, then... then that's quite strange.
Also, it's likely that as soon as you plug the drive into someone else's PC, it's gonna get the virus on it again - you can't stop that from happening. But, you can stop it from getting back onto tour PC. When you plug the newly infected drive into your PC, DO NOT DOUBLE CLICK ON IT! Right click on it, and choose open. Then enable viewing of everything again, and delete the autorun.inf etc
Whoa. Now my hands are sore and I've got a stiff neck. I think that should do it.