Can I create a popup/alert window/message when a SF page loads?

Is there a reason that you can't use a standard javascript alert? Here is a 'complete' VisualForce page using such a popup:

window.document.onload = new function(e)
{
if({!Contact.Some_Checkbox__c})
{
alert("Checkbox is True");
}
else
{
alert("Checkbox is False");
}
}
</script>

If the custom checkbox field 'Some Checkbox' on the contact record used in the URL (which would be "http://someinstance/apex/ThePage?id=") the popup will say 'Checkbox is True' and vice versa.