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=9.0.242.0, 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.

Thanks! 😀