You cannot open another window from the plugin.
The other option is if your plugin is registered post event. you can update some field in the enttity. After the event is finished, on the form load check if this field has value then open a account form (form load event will use javascript)
I hope this helps.
Amreek Singh Senior CRM Consultant CDC Praxa Sydney, Australia
http://mscrmshop.blogspot.com
http://crm2011usersettings.codeplex.com