Elk Cloner wasn't a boot sector virus
Mikko is wrong here - technically, Elk Cloner wasn't a boot sector virus. It was an OS infector. The virus didn't touch the boot sector. Instead, it modified the operating system (called, unimaginatively, DOS), which resided on the first 3 tracks of the floppy disk (after the boot sector). Unlike the MS-DOS, which resided in files (that had to be, however, in fixed places on the disk), the Apple ][ OS was not visible from the file system; it occupied whole disk tracks. There were some unused sectors on these tracks - this is where the virus put itself into, besides modifying a few instructions of the OS to make sure that its code was called. (There were legitimate - non-malicious - variants of the OS where the "unused" sectors were used to add various useful extensions to the operating system, like a line editor for the command line with command history. The virus would damage these if it managed to infect the disks containing them, but that wasn't really a problem, because these OS dialects appeared much later, when the virus was no longer widespread.)
The Multics cookie monster wasn't a virus, since it did not replicate itself. It was just a joke program or, with some stretch of imagination, a Trojan Horse.
The CHRISTMA EXEC can be called a virus (well, a worm really) only with some stretch of imagination, since it resided in a text script that the user was supposed to execute manually. That is, when you got it, and started viewing it, you saw at the beginning a bunch of commands for drawing a Christmas tree and some text that said "reading this is no fun at all, simply execute it" (not the exact wording). If you did execute it, some code at the end (which the sender hoped you didn't see after the many lines drawing the picture) re-sent the file to all your contacts (after drawing the promised picture). Kinda like the joke e-mail that said "Check if today is Friday the 13th., If it is, delete all your files. If not, forward this message to all your contacts."
And, of course, it wasn't a PC virus.