I have an ACER AX1700-u3700a that does not power on. Checked the basics; cable, power outlet is good....Opened the PC and noticed that when the power button is pressed, the CPU fan moves slightly then stops....I read in an ACER blog of a person having the same problem and he found out it was his video card....So I removed the video card (GeForce G100 512mb DDR2) and the PC turns on. QUESTION: Is it the video card or the Power Supply on its way out ?

Looks like you have already answered your question but the PS behavior also points to a marginal power supply. If you replace the video card and you still have the problem, the PS is next. I would recommend replacing the original (if needed) with one offering high ratings since the mfr.s like to cheat to save a few yuan as long as it is an ATX type and matches yours for outputs. The cost difference is often much less than the increase in power you can buy so I would replace with a unit rated at minimum 25% higher rating; it will run cooler and live longer.
These things are incredibly cheaply made and failure of them is common.

If it was an intermittent problem, like the computer would turn on 1 out of every 4 attempts WITH the video card in, then MAYBE it's a bad power supply. With everything you have said though, it definitely just sounds like a bad video card. I have burned out 2 video cards myself and had to replace them, so I wouldn't be shocked that you have a bad one because they seem to go out more often than power supplies.
Replacing either component can be pretty costly, so if you want to test the hardware before you go off and buy new hardware, I would try to borrow a working power supply of equal or greater wattage and try to run your computer with that and your current video card. To see if you can get it going, but I'm betting the video card melted somewhere.