I did something like this just recently, using an animated GIF.

I print out

<img src="rotating_thing.gif" name="progress">
[download]

when the process starts, and I print out

<script>
  document.images.progress.src='finished.gif';
</script>
[download]

as soon as the process has finished.

It at least gives the user something to look at while the process is going on.

Obvious question -- why does your process take so long? Why don't you ask us if we know a way to speed it up?