What do you mean by that?
Can you illustrate somehow i.e. what boot options are shown to you in BIOS.
Also.
Which version of ZorinOS have you downloaded e.g. Lite or Core?
I have seen items on other forums that say that whilst the AMD Turion 64 is 64bit, you may be better off with 32bit OS if you have little amount of RAM.
Did you confirm the SHA256 checksum for the ZorinOS download to confirm download was not currupted.