Hi 21seenu,
I would like to know if you have installed all the drivers after reinstalling Windows 7.
For the mentioned key combinations to work, Dell Quickset has to be installed. Refer the following link for the Vostro 1550 drivers page: http://dell.to/10LAAUJ
Select the operating system as Windows 7. Download and install Dell Quickset that is listed under the category Applications.
Regarding the issue with display resolution, ensure that the graphics drivers are installed. You may refer the following link that has instructions to install drivers in correct order after reinstalling operating system: http://dell.to/Ynnsnr
Please post the results.