Windows 7 won't reboot properly. I visited a site (p*&n) after antivirus expired. Didn't see anything weird going on. Days later when computer is started it goes to a black screen after account log in. Computer won't allow full instillation of antivirus in safe mode. Error message recorded "installation successful, antivirus/ scan failed to download" < or something similar. I am unable to activate any windows security as I'm being denied (in safe mode). Cmd prompt "sfc /scannow" entered, results "beginning verification phase of system scan. Verification 3% complete" results "windows resource protection could not perform the requested operation". Days before error and antivirus was active, the same command from cmd was completed successfully. What is causing this, I need to operate in normal mode, I am reporting from the possibly infected computer

You need a bootable antivirus CD. It sounds like you might have a virus that is preventing you from installing antivirus software. With an antivirus CD, this type of virus cannot do that since it won't get the chance to load on start-up. The more expensive options are something like Symantec; however, you can look on amazon.com for a cheap bootable antivirus CD, and see if that allows you to get rid of the virus. It could also be data corruption. Some of the bootable CDs that you can buy on Amazon will also check for and fix corruption. This is the cheapest way to go.

Which type of antivirus were you running when it expired?

Never run as an admin on any windows box. If you are an admin user make a new account that is an admin account then change your account to a standard user this will stop over 80% of infections. Install Microsoft Security Essentials OK I know it is not the Best but it is free and it is far better than most.
Now to get you up again if you have a data backup format and reinstall never "Fix a compromised system (you can never be %100 percent sure it is "Fixed" If you do not have a data backup you can use one of several Linux boot disk or USB to boot and save out a copy of your data. There are also some Linux bases utilities that have virus removal tools however once you are infected to the point that your boot sector has been infected there is no garentee that the infection is removed properly. Good luck and I hope you are backed up with a copy of recovery media