That sound is actually normal. Once the rack and pinion reaches its limit of travel, trying to force it any farther is putting maximum strain on your power steering pump. Its actually very bad for it. Holding it and making it squeak for more than 3 seconds may damage it.
If this sound is occurring whenever you turn at all, you may just be low on power steering fluid. Get some more.
The 2002 Lexus uses serpentine belts with an auto tensioner. So anybody who told you that you have a loose belt is a moron.
You may need to change a serpentine belt, but these belts last a very long time, just inspect the belts and look for cracks, separation, and feel for smooth spots on the inside. Their incredibly easy to change. Find the auto tensioner and crank it to the left with a socket and ratchet, preferably with a breaker bar. Hold in position and the belt comes off with ease.