Is your Windows 7 installation disc burned, borrowed, purchased from a store, or???
If you want to do a clean install, the general process is:
1: disconnect all peripherals, such as printer, external drives, etc. All you need is monitor, mouse, and keyboard. Disconnect any hard drives other than the one that will receive Windows 7.
2: Boot from the installation disc in the DVD drive, rather than from your hard drive. You do that by choosing the DVD drive from a menu or by setting the PC to boot from the installation disc in the PC's BIOS. Which of these have you done?
3: After booting from that disc, you should come to a screen asking you to choose a language. Shortly thereafter you will come to a screen asking "where do you want to install Windows".
At that point, you would delete all partitions and continue to follow the prompts.
Which of those steps fail for you? What error messages come up on screen?