How can I suppress the standard Windows 7 "What do you want to do" dialog when you plug a hard drive in to a PC? Is there a special file I can put in the root folder for example?
When the dialog appear with the choices you can check the checkbox at the top which says "Always do this action" and select either "Take no action" or "Open folder to view files".
If the above isn't a choice you can always set it at Control Panel:
Go to to Control Panel then search for Autoplay (conveniently you can use the search box at top right).
At top you can set default for file types. Scroll down and you'll find a list of connected devices. You can set default for each device.
You can set as Take no action or maybe Open folder to view files in Windows Explorer.
If that doesn't work, the last will be Group Policy Editor:
Press Orb button (start menu) and type gpedit.msc in the the box. Press enter and go to Computer Configuration > Administrative Templates > Windows Component > Autoplay Services. You'll see 4 options, and need the first one: Turn off Autoplay. Click on it and check Enabled, you can also choose to which devices you want it disabled. Note: the above is a the way to modify the registry key without dirty hands. Everything done in Group Policy Editor (gpedit.msc) is a registry modification.
If all that fails, then go directly to registry at: HKEY_CURRENT_USER > Software > Microsoft > Windows > CurrentVersion > Policies > Explorer... Well, you're on you're own here, try all the above first :)