What is the name of the processor? Is it a Pentium 4 or AMD Athlon 64 or something like that? If you don't know, google CPU-Z, download that and run it and it will tell you what type of processor you have.
Only two families of processors are x64 - the Athlon 64 family and the Intel x64 chip (can't remember the name right now), and anything else is considered i586.