Outlook offline folders error

Hi there,

Some of my users getting the following error while opening Outlook.

“The file x:\Outlook Archives\Outlook.ost cannot be opened”.. and after restarting the PC, it will work. One user complaining that his outlook sometimes getting hanged and it will not show the newly arrived emails. (when he opened @2.30pm..there was no emails..when he opened @4.00pm..there is lot of emails and many of them shows as arrived before 2.30pm.!)

we are using SBS 2003, Exchange Server 2003,Outlook 2007

Please help me to solve these problems.thanks