Exchange 5.5 running out of disk space

The hard drive on the server where Exchange 5.5 resides is running out of disk space. Most of the users connect to the server through the Exchange Server service in Outlook 2000. I had a few users that had lots of emails delete and or move those messages to a .pst on their hard drive. This did not free up any space on the hard drive. What do I need to do to free up some hard disk space? What will happen if the partition that holds the Exchange information fills up? Thanks.