1/80s at f/5 and iso100 comes out to about
EV11. That's equivalent to a medium bright overcast day, only you were shooting that indoors and at night. Even directly at a lightbulb is not going to get you up to EV11 for the proper exposure, and you're probably on matrix metering which is trying to balance the whole scene. So now you say that you need to drop it all the way down to 1/5s to get a good indoor exposure? 1/5s and f/5 and iso200 comes out to about EV6, which is a fairly well lit indoor area at night! My household indoor lighting is about EV5, and if I leave my camera parked at iso200 and f/5, then a shutter speed of about 1/5s is about what I'm going to get. So it sounds like your camera is working fine to me. Of course you're gonna have a SLOW shutter speed. The light is dim, the default lens is slow, and you have your camera's sensitivity turned all the way down. Normally in EV5/6 light I'm using a fast f/2 or better lens, iso800 or 1600, and a shutter speed of between 1/60s to 1/125s which is quick enough to freeze the motion of my 11 month old daughter. Or I can use a FLASH and the camera will work with just about any setting I put in with the limitation being having enough flash power. Why did your photos miraculously look OK when using a flash? Because the exposure system then had an element of control that you weren't messing up and was able to work around your bad settings with the flash power, which wasn't under your control.
I wasn't trying to be harsh before, but it was pretty clear that you didn't understand why using a faster shutter speed or a smaller aperture were making your photos go dark all while leaving iso parked at the base 100 indoors at night, which tells me you don't understand the basics of exposure, and that you didn't know how to read your camera's meter, and yet you're in FULL MANUAL mode. You may have read some stuff online and you may have read your camera's manual, but that's no substitute for real experience. You MUST know what you're doing to be using full manual mode, and most people who are brand new to photography don't. I bet if you put the camera on full Green icon automatic mode it'll work fine, and I doubt the camera is defective. This is why I said to read your camera's manual, and consider picking up
that book.
I'm sorry for not sugar-coating my original post, but stop blaming the camera. It seems to be working fine based on what you're saying. You're just not understanding why it's doing what it's doing, and for that you need to do some more reading (like that book) and get a little more experience before you jack your DSLR into full manual mode.

And the reason people all recommend that book is because it goes into great detail, is easily understood, very cheap, and so there's really no point in anybody here trying to reinvent the wheel.
If you don't want to buy a book, I find this thread to be excellent:
Canon DSLRs and Lenses 101