Yes! We can run different versions of Oracle on the same machine provided that Oracle_base_directory is same for both the versions.
Oracle_base_directory is the directory where oracle is installed like C:\oracle or D:\orant
Under the Base directory Oracle home directories are differentiated like
C:\oracle\oracle8
C:\oracle\oracle9
In short you can have different versions installed on the same machine if they are installed in the same directory..
try and post your result.