The first thing to do is to put the Windows 7 disc into the DVD drive and shut the computer off. Then press the power button and continually hit 'F2' and 'F12'
You should either see BIOS or the boot-menu appear.
From there, you can decide where to go in terms of making the DVD drive the number 1 boot priority, or just choosing it for the one time boot option.
If BIOS doesn't open AT ALL, it's most likely a CPU/Mobo issue.
Let us know what happens