On XP, though similar for other Windows: Right-click a blank area of the Tashbar, then go to Toolbars and check that "Quick Launch" is checked (if not, check it!) this may cure the problem. If not, go to Start>Programs (or anywhere else you might launch Outlook from) and drag it into position.
If you can't find Outlook, right-click the Start button and select "properties". Click the "Start Menu" Tab, and "Customize" the start menu (not the "Classic Start Menu"). Ensure that show Outlook as email on Start Menu is checked and then follow the procedure above.