You got a nasty friend there dude, anyways here's what you should do.
1. Relax and take a deep breath.
2. Start you computer and enter the bios by pressing del at startup.
3. In boot settings, change boot device to cdrom if you have a windows installation disc or removable device if you have the windows setup on a pendrive.
4. Also check that your hard drive is detected in the bios. THIS IS VERY IMPORTANT. If not, check whether the sata and power cables are connected properly. Remove and reconnect them if required.
4. Boot from the cdrom or pendrive.
5. Select the correct partition onto which you want to install windows 7, if there is no partition, create a bootable partition from windows cd itself.
6. Install windows.
7. Stay merry.