Here is some information from Outlook 2007 help, this may be what you need. It is a bit lengthy though.
Change where the Offline Folder file (.ost) is saved
When using a Microsoft Exchange Server account and you have turned on offline support or use Cached Exchange Mode, you can work with your Microsoft Office Outlook 2007 items regardless of whether a connection to the Exchange server is available. A synchronized copy of your Outlook items is saved in an Offline Folder file (.ost) (Offline Folder file: The file on your hard disk that contains offline folders. The offline folder file has an .ost extension. You can create it automatically when you set up Outlook or when you first make a folder available offline.) in a hidden folder on your computer. The folder is at the following location:
Windows Vista drive:\Users\user\AppData\Local\Microsoft\Outlook
Windows XP or Windows Server 2003 drive:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Outlook
How to view hidden folders
Windows Vista
In Windows, click the Start button, and then click Control Panel.
Click Appearance and Personalization.
Note If you are using Control Panel Classic View, double-click Folder Options, and then continue with step 4.
Click Folder Options.
On the View tab, under Advanced settings, under Files and Folders, under Hidden files and folders, select Show hidden files and folders.
Microsoft Windows XP
Click the Start button, and then click Control Panel.
Click Folder Options.
Click the View tab, and then click the Show hidden files and folders option.
You can change where the .ost file is saved, such as a different folder or hard disk. Because your data is on the Exchange server, it is not necessary to back up the .ost file. It can be recreated in the new folder or on another computer.
Note An .ost file is also used for the Microsoft Office Outlook Connector for Windows Live Hotmail. These .ost files cannot be moved.
Exit Outlook.
Click Start, and then click Control Panel.
Double-click Mail, and then click E-mail Accounts. If you are using the Control Panel Classic View, double-click Mail.
Mail icon location for 64-bit version of Microsoft Vista
If you are using a computer running a 64-bit version of Vista, in Control Panel, click Additional Options, and then click View 32-bit Control Panel Items. If you are using the Control Panel Classic View, double-click View 32-bit Control Panel Items.
Select the Exchange account, and then click Change.
Clear the Use Cached Exchange Mode check box.
Click More Settings.
On the Advanced tab, click Offline Folder File Settings.
Click Disable Offline Use, and then in the dialog box that appears, click Yes.
Note If you cannot click Disable Offline Use, this indicates the Use Cached Exchange Mode check box was not cleared in step 5. This check box must be cleared.
Again, click Offline Folder File Settings.
Click Browse, select the folder where you want to save the .ost file, and then click Open.
Click OK.
In the dialog box that appears, click Yes.
Under Cached Exchange Mode Settings, select the Use Cached Exchange Mode check box.
Click OK, click Next, and then click Finish.
In the Account Settings and Mail Setup dialog boxes, click Close.
Note The old .ost file is not deleted by these steps. To increase available hard disk space, you can delete the old .ost file.
Caution If you are using Outlook on two or more computers, a unique .ost file must be created on each computer. An .ost file cannot be copied to a second computer and then used on both computers.
