Dashboard quits when trying to launch widget from ~/library/widgets

Question

Hello,
My iMac has several accounts which work perfectly. However, my son just noticed in his account (his account is "standard" not "admin") that when he is using dashboard that the widgets that are preloaded (those in HD/library/widget) work fine BUT all those that he had previously added (i.e. those place in ~/library/widgets) do not launch. When he tries to run any of those he gets kick off dashboard back to the finder. I have tried eliminating the plist associated with Dashboard from his account but that did not work. Any suggestions?
Thanks.

Answer 1

Thanks for your help. However:
Yes, I have trashed both of those files from his ~/library/preferences directory and restarted. However, the problem remains unchanged.
Additionally, I have also eliminated his cache items (com.apple.dashboard.client and DashboardClient folders in ~/library/caches) associated with dashboard. The only change has been that this has wiped his preferences so now when I start Dashboard on his account I get the default widgets (e.g. weather, calculator, time and calendar... as you would when you have a new account). Those dashboard items work as they are from the HD/library/widget folder. Those widgets that he added (e.g. say cheese, etc) do not start and kick you out of dashboard if you try to start them.
Lastly, I have removed all the widgets that he added to his account and then tried to add them back. When I double click a widget install file from his desktop, I do not get the same "slow motion ripple effect" widget addition item that I get in the other accounts on the iMac nor the dialogue box that asks if I want to add the widget. After doing an attempted install of a widget from his account, if I start dashboard and hit the "+" I see the new widget item has an entry, however, clicking on this just gets me kicked out of dashboard again and I am back where I started.
Any more ideas???