Question : Problem regarding read only database

My Access XP DB is on a server. Quite often when I open my DB, I get a error message:
"The database '... ' is read-only. You wont be able to save changes made to data or object definitions in this database."
Incidently there are no lock files present when this error happens which could indicate that someone is accesing the DB. If I reboot the server, the problem disappears.
Could some one explain under what circumstances this error occurs ?? How to avoid it ?

Answer : Problem regarding read only database

Change the excel file to work in shared mode. In Excel goto Tools->Share Workbook->and check the box for "Allow changes by more than one user at the same time."