My Desktop XPS 400 (Win XP Pro SP2) will not complete the boot sequence. When the power switch is pushed, the Windows logo screen appears with the white rolling indicator, then the screen normally accessed by F8 appears. (This is the screen with the options: Start in Normal Mode, Safe Mode, Last Good Configuration, etc.) If Normal mode is chosen, the Windows logo screen and white rollling indicator appear, then it goes to a black screen with just the cursor. There is no response to any input. If Safe modeis chosen, the drivers list appears, then a black screen with "Safe Mode" in the four corners, and there it stays. Choosing  "Last Good Configuration..." results in the same dead black screen.

When the computer freezes up, in Normal or Safe mode, the power switch (steady green) is ineffective and I have to pull power to turn off the system. There are no front panel lights lit during that freeze. No beeps are emitted at any time.

I have verified (F12--System Setup) the boot sequence is correct: SATA hard drive is first, then CD-ROM. I tried listing only the SATA hard drive in the sequence. Same problems.

Obviously, I can't use System Restore and I don't have the Recovery Console installed. I have the Dell Reinstallation disc but not the MS XP Pro disc.

Shortly before the problem started, antivirus and antispyware scans were clean. I use mostly Firefox and occasionally IE7. Firewall is Comodo Pro.

After observing the above, I ran the Dell Diagnostics from the Dell Resource disc. System memory failures were reported. All other test results were good. I replaced the two 1GB modules with new modules of the same type. Dell Diagnostics reports all tests ( including processor with L1 and L2 caches, realtime clock, system timer, interrupt controller, system memory, SMBIOS, SATA hard drive, CD-ROM, video,etc.) good now. However, the same boot problems exist. What should I do now?