Slow Log on

We ran into a problem at the office lately. We have 16 poeple that log on and use roaming profiles. Not all 16 log on at same time. We may have 4 that are logged on at one time. We use win 2003 business server with domain. We use xp as the workstations. When users log on with thier user name and password, it gets to loading profile and it takes about 4-5 minutes for the profile to load. What can I do to speed this up? I also noticed the c: drive on the server keeps filling up and I have to take space from a storage drive and move it over to the c: drive. Any help would be great. Thanks