Windows Task Manager is used to view details about processes running on your computer. Since it can be used to terminate programs that are misbehaving or are viruses disguised in the form of harmless programs, most spyware and viruses disable it to prevent themselves from being closed through it. Some administrators also disable Task Manager to prevent users from closing important security programs like antiviruses and anti-malwares.
When changing the drive letters on your disc drives you can only change the drive to any drive letter after the hard drive. For example, because your hard drive is C: you can only change your CD drive to D: through Z:. If you have multiple hard drives or partitions and your last drive letter is F: you can only change your CD drive to G: through Z:.