Your options may be a bit limited.
First, I think I'd try a memory test. But for that, you need a memory tester - you can use either Memtest 86
or Microsoft Memory Tester
. You'll need to use another computer to make the disk, burning it from an .iso file that you download. Then, you need to get into your computer and run setup and once in the BIOS set it to run from the CD. Then the memory test will run - let it run quite a while - even overnight - to check for errors. This does not use the operating system, but runs on its own.
That's just in case the memory went bad.
If the memory test comes up with no problems, then you have to look deeper. Sometimes a drive or video card or soundcard will fail the hardware test and the operating system just won't start. At this point, I think I'd try using the XP disk to get into the repair console and repair the boot sector
The big problem is the lack of error messages. A blue screen would give an error, but it doesn't seem to be getting that far, so I'm thinking hardware. Since you can see the BIOS thing - I'd guess your video card has some functionality and the computer is trying to boot. But one piece of hardware, such as a drive or video card or memory that's gone bad can keep it from starting. The clue that makes me think it might be the memory is that you had the problem while in Windows for no apparent reason that started this all off.
This could also indicate that the power supply is going bad and has damaged the memory. The power supply fan is working OK, right?