A user asked how to make the first day of the week on the Vista Calendar gadget a Monday because they're from Bulgaria and used to Monday as the first day of the week. (Personally I think Sundays a strange "1st" day too, but that's a digression)
The first, most obvious method is by changing intl.cpl (windows gem/crystal/start thingy->control panel->regional and language options) user locale to Bulgarian. The calendar gadget is pretty good about picking up the user settings. Unfortunately as an HTML dohicky, it doesn't get notified about changes to the locale, so you'll either have to delete & readd the gadget or logoff/logon to get it to pick up the change.
If you don't want a bulgarian locale for some reason there are two other methods. One is to change the registry (windows+r run prompt, then regedit) key for HKEY_CURRENT_USER\Control Panel\International\iFirstDayOfWeek to "0". I don't recommend using the registry to change user overrides, and its not really supported because we might change it in the future, but I was a bit surprised to NOT find a way to change the first day of the week in the regional options control panel.
The other method would be to use the Locale Builder Tool, which should hopefully be released soon. (The beta version expired and the final version isn't quite up on the web site yet.) Using the locale builder (or custom cultures in .Net), you can create a replacement for en-US (or whatever your favorite locale is) and set the first day of the week to Monday.
Note however that changing the user overrides in the registry will impact every program running as that user, and making a custom replacement locale will change the behavior for the entire machine!