[henry]

You could try running Alcohol, click on the 'Virtual Drive' icon(bottom left and see whether it is enabled by looking at the 'Number of Virtual Drives' (top of the pop-up window).

If it is set to 1(or more), try setting to zero(click OK) then set to 1 or whatever number you require and see whether this shows up in 'My Computer' again.

The drive may still be enabled, but for some reason not show up in 'My Computer', if after trying above it still doesn't show, you can mount images form Alcohol by right clicking on the virtual drive, at the bottom of the window under 'Device' after running Alcohol.