Tj107 is right. you properly have a CPU that only supports 32bit OS.
all modern CPU's have 64 bit support, so your computer must be old.
A good site to download Lunix OS distributions are at the official site for the chosen OS it self.
So if you want to download Kubunu (my favorite btw) then go to www.kubuntu.org
In case you want ZorinOS, then go to www.zorin-os.com