The BIOS POST screen happens first thing, before the OS is loaded. Disabling applications from loading in Windows with msconfig shouldn't have any effect on whether you see the BIOS screen or not. POST stands for Power On Self Test. Basic hardware detection is one of the things done during the POST. If the BIOS is set to quick boot, minimal checks are done. If you turn off Quick Boot, more in depth checks are done. Failing hardware can cause a failed or prolonged POST/BOOT. One of the last things done is that the hard drive is ID'd and checked for an OS. Then command is transfered to the OS.
Don't get me wrong, using msconfig is a good idea when trying to figure out whats screwing with windows. In this case I just think there is more than that going on on your system.
Todays systems can boot from USB devices and I've seen quit a few threads where an external USB device prevented the PC from booting up in a timely matter or at all in some cases. I've also seem some pretty strange things happen when the power supply is flaky.