Need advice for potential long running process

[Lloyd Sheen]

I have a site which basically does the following:

Create a treeview with a folder structure (determined at design time).

When the user clicks a node the process will:
1. Determine if there are graphic fieles (BMP/JPG)
2. If so generate thumbnails if necessary (if they don't exist)
3. Present a table of thumbnails to user

The generate thumbnails could take a while as could the presentation of
them. There are some folders which may have up to 200 images. When the
user clicks the treeview node (I really hate that control and it's lack of
events), I will capture the folder from the QueryString, then generate if
neccesary (some will regnerate if the original graphic has been changed) the
thumbnails.

The page is not showing and there is no indication that anything is being
done except the flag flying in the corner. If the generation of thumbs
takes a while it is easy for the user to click another node but this click
does not interupt the generation. Is there some accepted method to show
some UI to indicate what is happening and to perhaps disable the treeview so
that the user will not keep clicking?

Lloyd Sheen