If you entered either of the values you give, the cell doesn't contain a date, it contains a string. You can't change as string just by changing the format!
Excel tries to guess your data type, and that can be really confusing. If you enter some kind of data with an obvious type (date, time, date/time, number) it can figure that out. If the type isn't obvious, the it assumes you're entering a string.
If you enter an obvious date/time value like 14/Nov/2011 10:53:44
(notice the space between the date and the time) it understands that you're entering a date. But 14/Nov/201110:53:44
(no space separating date and time) confuses it, causes it to record a string.
Re-enter the date-time value correctly, and you'll have a date-time value. (Actually, you'll have a number formatted as a date-time, but let's not nitpick.) Then you can control the way it's displayed.