Is there any software to provide a sidebar in XP like that in Vista?
I have tried docks, but not good enough.
Also if there is any application for setting an alarm at a specific time?

Yahoo's widgets (http://widgets.yahoo.com) are pretty popular and there are thousands of widgets, including alarms.

You can also use Google's Desktop (http://desktop.google.com), which is the vehicle for their desktop search. However, if the search capability is annoying, you can disable content indexing in the preferences. This will leave you with the sidebar, which like Yahoo Widgets, has a huge number of widgets you can add to the sidebar. You'll find plenty of alarms to choose from.

I like desktopsidebar. I think there is an opacity setting in it as well, because mine is see through. If its not a setting, then it is certain skins that allow it.

I've taken a liking to Google's Desktop. Lot's of widgets.