Virtually any machine is ready to use Excel, but it does assume you have a
legitimate copy of Excel to load onto it and then use. I don't know that you
can download Excel, other than I think I saw a downloadable trial version of
2003, so not sure what you mean when you say you downloaded 2000. If you didn't
purchase a copy of Excel with your machine, or have a copy bundled with your
machine, or purchase a legitimate copy elsewhere to be loaded onto your machine
after purchase, then you are probably out of luck and will need to purchase a
copy.
You do not need to purchase any other software to use Excel other than Excel. I
assume you either have a PC or a MAC and each have have their own versions of
Excel/Office for purchase, and in neither case do you need to purchase any other
software in order to run it.
I would clarify with your supplier whether or not a copy of Excel was included
in your purchase, and if the answer was no then you will need to purchase a
copy.
The only caveat to this is if you only need to view Excel sheets rather than
work with them, in which case you can download the free Excel viewer from
Microsoft.