The Intel(R) Boot Agent utility will not work because the utility will only work on adapter settings. On a built-in network connection you need to use the manufacturer's utilities or BIOS settings. Disabling the boot to network (or similar option) in BIOS works in most motherboards. Since it is Dell's motherboard, they would be the only ones that could fix any bug in the BIOS.
Maybe your settings are not saved properly. If the motherboard has a reset option, you could try setting everything to defaults. Then you could try again to disable network boot.
Another thing you could try is to go to an earlier version of the BIOS. Unfortunately, the last couple of versions added support for new CPUs, so if your CPU is not supported in an older BIOS version you won't have that as an option.
A new motherboard gave the same problem to my Dell Studio XPS 435mt i7.
The solution was to enter the BIOS Setup (F2 at boot) and then Load the Default BIOS Values (F9).
This did change the Boot order to
1. SATA (the hard drive),
2. CD/DVD and
3. USB:Generic- Compact.
It may be noticed that before this reset, the unwanted net drive was one of the Boot Device options, but as I can recall not the "USB:Generic- Compact".
How ever, after the reset. It was the other way. No net drive, but the USB:Generic Compact.
Saving the new Boot Settings made my Dell Studio XPS boots nice and fast again.