One cause of virtual drives not dissapearing is when you uninstall one of the programs whilst you still have a mini image (or full image) mounted! That is the reason why you cannot reinstall it! It is conflicting with the virtual drive which is still there.
Go to the Alcohol Soft knowledge base (help page) they have a fix listed there for your problem. I dont know the link of hand but you should of tried there even before you posted your question here.
