The reason you cannot sometimes delete a virus is because it is running as a hidden process when Windows loads up or it is actively running on Windows and cannot be deleted because it is being used. When you boot to safe it does not load up (most of the time) and then you simply unhide your files and find it, then delete it. This is if you just want to try booting to safe.
If you want to add the infected drive to another system to do all this then make it the slave and add it. When you boot up Windows will assign it a drive letter. As long as it is the slave you should be OK. The system will boot to the master (usually C\).
Just to answer your question tough you can assign drove letters through Disk Management. When you RC My Comp and select manage one of the options is Disk Management. You can assign them from there.
Fun stuff Eh?