Holy cow, I wrote a book!
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.