A virus would not prevent BIOS access. A boot virus replaces the normal boot code with the virus so it loads into memory before anything else could be in place to stop it. However, they may not still be able to start the OS. In the past, many boot viruses attacked floppies, since they could simply show the typical "This disk not bootable" message, then the user would continue to load the OS from the HD with the virus resident. Now that people don't use floppies, I'm not sure of their infection plan.