I've had this problem for a while. My Toshiba P205d laptop fails to boot if the power plug is installed. If I remove the plug and power on, I can insert the plug only after I start getting the vista loading bar at the bottom of the screen. It's the only issue. It did it with a HDD, and I replaced it with a SSD. I also bought a replacement battery but same thing. It must be either a boot thing or a hardware failure. I should prob check the BIOS settings also.
On rebooting, I'll get an option to repair the boot, and it may work for a couple of times, but eventually returns to doing the same thing, so I just boot up with the plug out until I see the vista loading graphic.