Intermittent booting problem with 8400

I am having a number of intermittent problem with my 10 month old Dell 8400 (3.4GHz)

 

These are;

1. When booting up the it will display the Windows XP logo and then just before the logon screen appears it will go back to the DELL bios screen and attempt to boot again. It then goes into this cycle refusing to start Windows XP
2. When booting up the it will display the Windows XP logo and then just before the logon screen appears a message box appears with the message "The endpoint format is invalid". I press the OK button and it restarts again from the Dell bios screen, and then behaves as point 1.

3. The system will start correctly but any USB devices refuse to work and the Device Manager reports problems with the device

4. The system has "Blue screened" after booting into Windows XP.

 

I have tried to diagnose/fix the problems by.

1. Running a chkdsk on the hard drives, no problems were reported

2. reinstalling Windows XP, this appeared to resolve the problem but it came back after a few days

3. Remove and re-seated the RAM in the motherboard.

4. Run a full virus scan using Symantec's AntiVirus, (which I keep up to date)

5. I have the latest Bios A08 and updated the Intel Chipset drivers.

6. Run the Dell diagnostics, they report no problems.

None of the above has resolved the problems. The only thing I have found to fix the above problems is to enter the bios and turn off "Hyper threading" on the CPU, it then appears to make the PC stable, but as the above problems are intermittent, it may be coincidental. I believe the problems is CPU or Motherboard related.

Has anybody got any ideas as to the cause of the problem.

 

Thanks