IE Locking Up

I have a user was using IE 4.0 and whenever he opened his IE brower it locks up for up to 5 mins. Then he can work normally. I had him update IE to IE 5.5 with Sp1 and he is still have having the problem. All other programs on his machine are running fine. Has anybody ran into this problem, or know what I could possibly try next (besides rebuilding the PC)?

-Adam