Is it a custom build or a premade by a brand? Did you recycle any parts from your old computer?
Assuming its brand new, and not sure what the electrics are like in your house, it wouldn't hurt to try a different outlet. I know in my apartment for example, there's certain outlets they told us had a dedicated fuse, and thus could plug in our window air conditioners in. Maybe your dealing with something similar?
Let us know what kind of build you have.