I can't tell you exactly (don't own a Sony) because BIOS setup differs between manufacturers, but they're all similar.
Chances are the mouse won't work at that stage so you navigate round the page using the keyboard arrow keys.
Along the top of the BIOS setup page you should see a line of menu options like General, Advanced, Boot, Power etc.
Highlight the "boot" and hit enter.
You should be in a menu page that lets you choose the boot device priority. Fiddle around (there should be advice on the page) until CD is before HDD then put your DVD in the tray and choose the "save changes and exit" option (probably F10).
The PC will reboot into the DVD and ask you to "press any key". Do so (just this once) and follow the advice here
Installing Windows Vista - Guides - NeoSmart Technologies Wiki
(the example is Vista, but W7 is just the same)
When you get to the "drive options (advanced)" section, choose to completely overwrite the dodgy partition (or delete everything and let W7 start with a completely blank HDD if there's nothing on there you need to keep).
During the install the PC will reboot 2 or 3 times and you'll see the "press any key..." message again.
Don't do anything.
Pressing a key will start from the beginning again, so just leave it to get on with the install till it's done.