To search in Outlook 2003, use the Look For (search) feature at the top of the Tasks view. Enter words here to search for a task. (If you do not see Look For above the word “Tasks” in the task list header, press Ctrl+E to make it appear.)
Make sure the Search In box says Tasks (it already should). Type a word or phrase from your task, then press Enter. Matching tasks appear in the list below. When done, be sure to click Clear to the right of the Look For box to clear the search and restore your full task list.