I haven't had to mess around with XP boot options for a while, but there should be a way to get it to show you which drivers it's loading (I think in 2000 this was called Verbose Boot, but the only way to do it in XP is to use Safe Mode). If it's hanging in a particular driver, that might give you a clue.

Also, what are the temperatures like? I was having a similar problem on the occasional boot thanks to a broken fan on the graphics card which then required the entire system to be powered down for a number of minutes before reattempting booting. Adding a new fan fixed things (go go gadget cable ties).