Why is my filesystem being mounted read-only in linux?

perhaps it is because the disk is unclean, try changing:

/dev/sda1   /         ext3    defaults    0 0

to

/dev/sda1   /         ext3    defaults    0 1

or at least do an fsck and then reboot

You say it's a VirtualBox machine... Does the VirtualBox process have write privileges to the datastore on the host?

mount -a remounts everything as described in /etc/fstab. If this is not behaving as expected, there may be some output in syslog. Check and post here if there is anything relevant.

If your are running Ubuntu then try first removing ntfs-3g and then install by running - "aptitude install ntfs-config" This usually fixes it. (The problem is that if you installed some other version of ntfs driver (e.g. additional options og parted) then it does not have write capability.)