The 64 bit edition was written for 64bit PC’s. If you did not remember purchasing a 64bit PC, then yours is most likely to be a 32bit. If you open the my computer icon, then double click your c drive and you see 2 program files folders, one of which says program files x86, then you have a 64bit OS. If you only see one program files folder, you are running a 32 bit OS and you most likely have a 32 bit PC, which is more than adequate for most users.
Here is a <a href=”http://www.microsoft.com/windows/windows-vista/compare-editions/64-bit.aspx”>more in-depth explanation.</a>