How can I get to DOS Command prompt???? Please help.

As you can imagine I have another PC downstairs (sisters) which she has just restored due to a virus. She uses her TV as the monitor and the problem is she can't remember where she was to type in a command other than it was at a DOS command prompt. mThey are trying to do so thru the windows based DOS command prompt. How do you get into DOS prior to windows loading. I know ther is a function key but not sure which one it is. If you can help it would be greatly appreciated. Thank you. Sister is driving me crazy LOL