[Stereodude]

if i buy a celeron chromebox and install kodi, will it run well or not?

Yes, it runs well. However, you may have some corner use cases where it doesn't since you didn't elaborate on how you want to use it.

can i install windows 7 into this pc or not?

I think Windows 7 is a no. Windows 8, yes, but that's not really what it's meant for. If you want to run Windows buy something like this: http://www.frys.com/product/8646880