Alcohol messed up my drive mappings as well, it creates its virutal devices before the drive letters are assigned to the physical devices.
First thing to do is to change the virtual device letter to something other than D, if that is what it is now. You should be able to do that by right-clicking on it and messing with the properties inside of Alcohol.
To set a fixed drive letter in Windows XP, go to Control Panel -> Administrative Tools -> Computer Management. Expend the Storage section and select Disk Management. Find the device you want to modify in the bottom right pane, right click it, and select "Change Drive Letter and Properties". Voila.