One of the problems that occurs when reloading a new OS onto a machine and not using the manufacturer restore disks is that sometimes a lot of the drivers that the machine needs are no where to be found on the new installation disks... If you were to ask me about this before you reformated, I would have suggested that you use the restore disks that came with the machine then upgrade the OS as you see fit. Since the formatting has been done, it looks like the problem is that you don't have the right driver for your monitor or video card.
You'll need to go the website of the manufacturer video card and monitor and download the appropriate drivers, install them on your machine... and once you do that you should be able to change the resolution.