You have a corrupt mail folder most likely.  In Outlook go to Help > Detect and Repair and run this and see if it helps. 

There is a special advanced way to fix your mail folders if the detect and repair isn't working.

Make a backup of the PST file before running the Inbox Repair Tool because if it isn't successful it can corrupt a mail file further.

The Inbox Repair Tool is a Microsoft utility for repairing damaged pst files. It is located in the C:Program FilesCommon FilesSystemMapiC3NT directory if you are running Windows 2000 or Windows XP. It is called scanpst.exe. Other versions of Windows also include the Inbox Repair Tool, but it can be located in various directories. So can you do a scan of your system for this file and run it.  Then follow the directions.  Additonal information can be found here.  http://www.slipstick.com/problems/scanpst.htm

Outlook Express

In Outlook Express,  try to delete the Spam folder.  First, make sure you don't have any important email in the Spam folder.  Then, delete this folder by highlighting it in the folder menu and selecting the Delete key (Del).  Then close Outlook Express and reopen it.  Then Create a new Spam Folder the way you normally would in Outlook Express by right clicking the folder menu (right-click on local folders) and select New Folder.