Generally there is no place to "legally" download any
commercial MS software.
If this was a converted trial version that you downloaded
from Microsoft, you should have copied the download file to
a CD. It is up to you to protect downloaded programs, not
the supplier to allow unlimited downloads.
If this was preloaded on your computer, it is up to the
computer manufacturer to provide some way to return the
computer to its delivered state. Check with the
documentation or manufacturer of your computer to see if
they will provide it if you can't find a copy.