Re: virus scan: online vs offline
Running the scan offline would ensure that no services are running, and that no programs require termination to quarantine any viruses. However, as Gibson mentioned, the hardrive you hook your computer up to is vonerable to the viruses on the infected drive. Therefore, if you are having a seriously troublesome virus, I would recommend hooking the drive up to a computer that does not have any personal or important files, and most preferably disconnected from the internet.
Otherwise, if it is simply a pesky virus, you should scan using the HD's original host. Though, normally if you are performing a manual scan you should not be doing other tasks, or shouldn't expect normal performance at least.