XP SP 3: hangs on boot

Suddenly I'm unable to boot up in any mode.

On starting up, after seeing the Dell bios screen I get the "We apologize for the inconvenience" screen.

In safe mode it gets to the DOS-like screen that lists 24 drivers (the one with the lines that read "multi(0)disk(0)....") and it goes nowhere.  I ran the hard drive diagnostics and the HDD passed.

If I try to "Last Known Good Configuration" or  "Start Windows Normally" it hangs on the black Windows splash screen.

Any ideas as to whether this is an OS issue?

Dell Dimension 8400; XP SP3. I run ESET NOD32 daily and Spybot S&D 3X per week. Boot drive defragged automatically with Diskeeper 2009 Professional. This installation is 6 months old, as is the boot drive, a Western Digital Caviar Black 750 GB.

Any troubleshooting advice would be appreciated.

Thanks.

DG