I am assuming your copy of Win 7 is on a CD/DVD. It should be sufficient to turn the computer on and let it go as far as it wants, it doesn't matter. Open the CD/DVD drive, put the install disc in and start the computer up again. You should then see a message on the screen 'Press any key to boot from CD', literally press any key ( I usually hit the space bar - it's the biggest ! ) and the installation process should start running.
If it doesn't, it may be because your BIOS is not set up to boot from the optical drive first, Again, this is fairly easy to change. Immediately after you start your computer, you should see a screen probably mentioning HP with some text on it. As soon as you see this screen, tap the 'Delete' key a few times and this should take you into BIOS setup. If it doesn't, re-start and look at the text on the screen, one of the lines will say 'Key 'X' or 'Y' or 'Z' to access setup'. Re-start and press this key, and you should be in the BIOS setup. If you are, along the top of the screen you will see various headings, among which will be 'Boot'. Use the left / Right arrows to highlight this and press 'Enter'. This will take you into this screen. Use the Up and Down arrows to select the command 'Boot Priority' and press 'Enter' again. This will open the command and you will be able to check the sequence.
It should be 1st - Optical drive; 2nd - hard drive. If it isn't, follow the instructions on the right hand side of the screen to adjust the sequence. When you have it correct, press 'Escape' twice and you should have a box pop up which says 'Save changes and exit' - 'Yes / No' Select 'Yes', press 'Enter' again and you should now be able to get the computer to boot from the CD/DVD when you put it in the drive as above.