Screen Resolution after Upgrade help please

Hi I am totally new to the forum and Windows 7 as of yesterday.

My problem is I had an HP Pavilion Entertainment PC (im not quite sure the model number) and I used an external HP monitor with it when i ran vista.

After i used an update disc to upgrade to Windows 7 Ultimate my external monitor now screws up the resolution. It fits the whole screen to it but makes everything look stretched out and text is fuzzy/images are stretched etc... BUT when i unplug the monitor the laptop screen reformats and works fine.. but when its plugged in the laptop screen has black lines on left and right..and the monitor looks bad.

I was wondering if anyone could please help me solve this as I would like to use my external screen.. THANKS!

If anyone can help that would be awesome! I am confused as to why Vista would work fine but when i used a new WINDOWS 7 it wont? ( i though vista was the bad one)

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