You can try out different things, before concluding to any big issue:
1. Try to copy any text with mouse and then paste it with mouse itself. If this works then there must be some problem in your keyboard or keys itself.
2. Forward or reply any email and then see if the same problem persists. If yes then follow the mouse trick again.
3. Restart your PC and Outlook Express. This can also happen if Outlook Express was not closed properly i.e if power went off or your PC restarted automatically..and OE wasn't closed down normally
