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If you have problem running Packet Tracer 5.1 on Windows Vista Ultimate Edition by which you see the following problem,

1. Installation is successful
2. When  you run the program, the application interface do not show up on the screen
3. But when you check the processes on the Task Manager, you can see the process Packet Tracer

I manage to resolve the problem by doing the following,

1. When you run the program for the first time, normally Vista will pop up a message asking you to block or allow internet access to the program. Make sure  you allow access.
2. If you have anti-virus installed ( in my case McAfee ), McAfee is the program that asked whether to allow internet access or not. I accidentally block the program.
3. To unblock it again, locate the McAfee icon on the task bar and right click and choose Open Security Manager
4. Next, on the left pane, select Home
5. Under the Home section, click View Recent Events
6. On the left pane, you will see all the recent event including your previous action of blocking the program
7. Click the event which shows you blocked the program
8. Below the event pane, you will see another smaller pane which says I want to…
9. Click the arrow line which says Allow Access

Now, you should be able to launch the program. Have fun with your networking skills…