I recently purchased a Umax Astra 2500 scanner and had similar problems at first, and kept uninstalling and reinstalling.
I now have short cuts to the Vista Scan and to Presto Page Manager on my desktop.
What works for me is first to open the Vista and down at the bottom right of the screen you can see it loading and showing successful connection. After this I can go to acquire in almost any supporting programme.
I have found that certain scanner settings, especially black and white give this problem so I stick to greyscale or colour