6 minute start up

My system has slowed significantly in the last two months. Startup takes a long time after the XP logo screen appears. I have added and subtracted a few programs latelybut toomany to list. I have a 500 meg system wth a 500 mhz processor. Also have lots 120 gb of disk on three drives. What are the categories of things that the process is going through and which are doing me in?