If you can, you should scan for viruses and spyware since it obviously installed something to your computer. Whatever it was should be eliminated first or it will just keep messing up your system. Then concentrate on restoring your Outlook Express.
If you have a backup of your .dbx files then you can restore them.
-see 1st link below for steps-
If not, then try compacting all your Outlook Express folders.
1. On the File menu, click Work Offline so that no new messages will be arriving.
2. In the Folder list, click on Outlook Express so that no e-mail or news folder is open.
3. Close the Folder list by clicking the X in the upper right-hand corner of the list panel. Alternatively you can click Layout on the View menu, and then clear the checkbox for "Folder List".
4. On the File menu, point to Folder, and then click Compact All Folders. Do not use your computer until the process is complete, which might take several minutes.
5. If an error occurs, close Outlook Express, re-open it, and begin the process again from the top. If the same error occurs again, close Outlook Express, restart your computer (or log off and then log on again), open Outlook Express and then begin the whole process again.
If compacting does not correct the corruption, you will need to move all messages out of the damaged folder (if possible), close Outlook Express, and then delete the associated DBX file for that folder. If you cannot open the folder, move the damaged DBX file out of the Store Folder into a new folder you create.