XP Freeze Up!

Here it is: I have an HP 4150B laptop. Runing XP Pro w/256 MB ram and a 450mhz cpu. If the wall power cord is not plugged in when it boots up, the PC freezes after the log in screen. If I leave it plugged in until after everything loads, it willrun fine on battery power. I have the most recent BIOS for this model. I have toyed with the power management options. Im stumped.