Normaly, you don't need to enter the BIOS settings to reinstall the operating system. You only need the installation medium, normaly a DVD for Windows 7.
Start Windows normaly, insert the Windows 7 installation DVD in the DVD drive. It should start automaticaly, or you can start it manualy.
Follow the on-screen instructions.
To reset the BIOS password:
The procedure is brand and model specific. You provide neither.
Is it a second hand computer? Is it locked? If so, the seller must give you any needed password, or remove them all.
If the seller can't remove the actual passwords and can't give them to you, it's probably a stolen computer.
You need to read the user manual of the motherboard to find if it have a BIOS reset jumper, contacts or swtich, where it's located and how to use it.
Using the wrong jumper, contacts or switch, you risk to cause irreparable damages to the motherboard forcing you to replace it, probably damaging the CPU, maybe the RAM and some other components.
If you don't have a printed copy, you can get the PDF version from the motherboard maker's web site.