I think that with Vista it's probably best to install somewhere other than the default (i.e. don't put it under Program Files). This is because it creates all its data files there and this is a bit naughty under Vista.

To help me track down the weird data data problem, could you please:

1. Stop Cumulus.
2. Edit Cumulus.ini, and insert a line in the [Station] section:

Logging=1

3. Start Cumulus, run for a couple of minutes, stop Cumulus.

4. Send me the debug.log file (to steve@sandaysoft.com)

Many thanks,
Steve

Edit: You're not running Easyweather at the same time, are you?