Probably fake.  It would be sooo easy to make a program that apears to be hacking your computer, when in fact it does nothing at all.  And it does appear to be made in Basic, due to the Basic menus, and the run indicator.  And unless the get(, getCalc(, and send( commands are a lot more advanced than I thought they were, this is impossible in basic

I have no idea, as I am unfamiliar with how windows treats USB devices when it is on the log-in screen.  If it completely ignores them, then i would say that this is impossible, because there would be no way for even the most advanced virus to get into the system.  However, if windows does do some sort of autoplay, or moniters USb devices durring that time, then yes,  I would say it is possible.  Although very very very unlikely. 

The size of a virus that can crack windows probably can't even fit onto a calc

If Autoplay is enabled for USB devices, then you could probably use an ASM program to simulate an external USB drive and execute arbitrary code. Maybe.

The video is most definitely fake, however it is not impossible to get into an XP user account using only your 84+/SE.

You can write a Flash application which simulates a USB flash drive and a boot floppy image (a special one containing one of those "reset the XP Administrator accounts" bootable floppy disks that are available on the internet) and connect it to your computer via direct USB.

From there, your computer could boot off the calculator and you could run the program from the "floppy". That would let you reset the administrator account.

From there, you can simply log onto the XP Administrator account, or you could use another Flash application which simulates a USB keyboard and have it "press" the necessary keys to log in.

These Flash applications are certainly possible and I started work on it some time ago, called periph8x. It can make the calculator show up as a USB flash drive or USB keyboard. Unfortunately, what I never got to work was booting off the calculator. USB initialization seems to work differently when booting from the device as opposed to being in Windows, and I never solved that problem. But if I had, then yes, this is certainly possible (this video was partially my motivation for attempting it).