I know this is an older post, but we just ran into a similar problem and "googling" the issue, this was only one of two responses that came up.
We received the following error (SQL Server 2005 Std SP4, 9.00.5069.00):
The task with the name "Execute Process Task" and the creation name "Microsoft.SqlServer.Dts.Tasks.ExecuteProcess.ExecuteProcess, Microsoft.SqlServer.ExecProcTask, Version=18.104.22.168, Culture=neutral, PublicKeyToken=89845dcd8080cc91" is not registered for use on this computer.
Upon further investigation, we discovered the server was waiting for a reboot due to Microsoft updates. Upon re-boot, it corrected itself.
One word of caution... if you are not careful the "Execute Process Task" may be deleted, so just "back out" of BIDS/SSIS, then perform a reboot.
Argue for your limitations, and sure enough they're yours (Richard Bach, Illusions)