Many (most) new PC's come with a restore option on the hard drive that will restore the PC to the original factory condition. On Dell's that is F8 at power on. Other manufacturers have different methods. Many PC vendors supply a procedure (usually an icon on the desktop) to create discs that can be used in case of a failure or corruption.
Best to check with the PC Vendor to see what is avaiable or required for your specific PC. You can download a Windows 7 installation file, create an "iso" DVD (bootable DVD) and use that in conjunction with the Windows 7 Product Key. That will install Windows 7 but it won't install all the needed device drivers
and you will still need to download those from the PC vendor. MS Office is a separate issue, if you have the product key for that you can download office and use your product key. If not that may be gone, if you got it with the PC from the PC vendor contact them about it.