You can download a trial version of Office Standard Edition at
http://www.microsoft.com/office/trial/default.mspx and then buy a
license online to convert it to a full version. This is the only
downloadable format, and there isn't any download of just Word.
Frankly, I think this is a bad idea if you know you're going to
purchase anyway. You'd spend a very long time downloading, and then
all you'd have is a file on a disk that's going to crash someday. To
be safe, you'd have to burn it to a CD or DVD for backup -- so why not
buy the CD to begin with?
Check one of the comparison shopping sites such as
www.pricewatch.com
or
http://www.pricegrabber.com. Just be aware that really low prices
are usually OEM licenses, meant to be bundled with hardware and not
eligible to be upgraded again (among other restrictions).