I have recently experienced a similar problem after installing a new sound card. Even though the Device Manager showed the driver was installed properly, the computer would only boot in safe mode.
After several reinstalls of the driver, Win XP finally got it right. I was able to boot normally after that.
I don't know if this will help, but I thought I would share the experience. Sure sounds familiar.
Have you installed any new drivers recently?
If so, do all work properly? Sound, video, modem, NIC, Printer, or any other new hardware?
Any other noticeable symptoms or irregularities?
I'm having difficulty booting XP Pro. As Windows begins to load, I'm given a number of menu options to choose from
Safe Mode with Networking
Safe Mode with Command Prompt
Last know good configuration
Start Windows Normally
I've also tried a boot disk as configured per the instructions on Microsoft's support page
Nothing has worked and the sytem cycles after 30 seconds and reboots with the same menu options.
The system was working properly with all drivers updated on a regular basis. No new software or hardware was added recently.