Did you try and run disk cleanup and/or defrag on the system?
Is this an ISP POP3 account or an exchange account?Try removing your outlook profile and creating a new one. If this is a POP3 account try repairing the .pst with scanpst. It would be helpful to know what you have tried so far. Uninstall/re-install office if creating a new outlook profile does not work. Was it working before and now does not work? Please provide more details. Thanks.