import re

def sanitize_filename(filename: str) -> str:
    """
    Sanitize a given filename by replacing characters that are invalid 
    in Windows file paths with an underscore ('_').

    This function ensures that the filename is compatible with all 
    operating systems by removing or replacing characters that are 
    not allowed in Windows file paths. Specifically, it replaces 
    the following characters: < > : " / \\ | ? *

    Parameters:
    filename (str): The original filename to be sanitized.

    Returns:
    str: The sanitized filename with invalid characters replaced by an underscore.
    
    Examples:
    >>> sanitize_filename('invalid:file/name*example?.txt')
    'invalid_file_name_example_.txt'
    
    >>> sanitize_filename('valid_filename.txt')
    'valid_filename.txt'
    """
    return re.sub(r'[<>:"/\\|?*]', '_', filename)