How can I tell if I have the 64-bit edition of Windows?
Answer: Your wallet is empty. Seriously, there is no way you bought an Itanium by mistake. They are expensive machines: The entry-level workstation available from HP (who co-developed the Itanium with Intel) goes for over $3000 and the entry-level server is over $13,000. And in addition to paying for the computer itself, you probably had to install a custom air conditioning system for your building to keep it cool.
If you still aren’t sure whether you have one, go to Help.About in Explorer. At the top of the About box, it will say “Windows XP 64-Bit Edition” if you have it.