"I want to be able to dual boot into XP (and then upgrade one OS to Win 7)"  There is no upgrade from XP to any version of WIN7.  Its a complete Reinstall.

XP without service packs and a specific sysprep registry entry does not support 48 bit LBA.

NEVER EVER Been able to F6 Load drivers from USB Floppy only from Onboard Controller attached floppy.

 How to enable 48-bit Logical Block Addressing support for ATAPI ...

By default, the original release version of Windows XP Home Edition and of Windows XP Professional do not have 48-bit LBA support. Your computer must meet the following requirements to use 48-bit LBA ATAPI support:

• 48-bit LBA-compatible BIOS.
• 137-GB hard disk or larger.
• You must have Windows XP SP1 installed.

For the original release version of Windows XP Home Edition or of Windows XP Professional, you can enable 48-bit LBA for testing purposes. To do this, set the EnableBigLba registry value to 1 in the following registry subkey:

Warning Data corruption may occur if either of the following conditions is true:

• You use this registry value to enable 48-bit LBA support in the original release version of Windows XP Home Edition or of Windows XP Professional.
• You install an earlier version of Windows on a disk partition that was previously created by a 48-bit aware operating system, such as Windows XP SP1. And that disk partition is equal to or larger than the current addressable limit of 137 GB.

Note The previous registry setting is ignored in Windows XP SP1 and later. If you try to enable the 48-bit LBA ATAPI support in the original release version of Windows XP Home Edition or of Windows XP Professional by editing the previous registry setting, and you did not meet the minimum requirements, the following issues may occur:

• The registry value EnableBigLba is disabled. If you have a 48-bit compatible BIOS that can support a hard disk that has a capacity that is greater than 137 GB, only the first 137 GB of the hard disk are addressable. The rest of the hard disk is not used.
• The registry value EnableBigLba is enabled, but you do not have a 48-bit LBA compatible BIOS and the capacity of the hard disk is not greater than 137 GB.
  
  If you enable the 48-bit LBA ATAPI support by editing the registry setting, but you lack both a 48-bit LBA compatible BIOS and a hard disk that has a capacity that is see comment than 137 GB, you have not changed the system. The hard disk continues to function as a standard hard disk.
• The registry value EnableBigLba is enabled without a 48-bit LBA compatible BIOS, but you have a hard disk with a capacity that is larger than 137 GB.
  
  If you enable 48-bit ATAPI support in the registry and you have a hard disk that has a capacity that is see comment than 137 GB, but you do not have a 48-bit LBA compatible BIOS, only the first 137 GB of the hard disk are addressable. The remainder of the hard disk is not used.

To enable 48-bit LBA support by using an unattended installation with the Microsoft System Preparation (Sysprep) tool, follow these steps:

1. Copy the following text into Notepad and save the text as the 48bitLba.inf file:

   [version]
   signature="$CHICAGO$"
   SetupClass=BASE
   
   
   [DefaultInstall]
   AddReg=48bitlba.Add.Reg
   
   [48bitlba.Add.Reg]
   HKLM,"System\CurrentControlSet\Services\Atapi\Parameters","EnableBigLba",0x10001,1

2. Create a file named Cmdlines.txt that includes the following lines:
   
   [Commands]
   "rundll32 setupapi,InstallHinfSection DefaultInstall 128 .\48BITLBA.INF"
3. Locate the Sysprep\I386 folder in the Sysprep image, and then create a $OEM$ subfolder in this folder.
4. Copy the 48bitlba.inf and Cmdlines.txt files into the Sysprep\I386\$OEM$ folder.
5. In your Sysprep.inf file, add a key named InstallFilesPath to the [Unattended] section. This key must have the following value: InstallFilesPath = "C:\sysprep\i386"

To add the previous settings to the Images folder that was created with the Riprep.exe program, follow these steps:

1. On the remote installation server that contains the Riprep image, create a Sysprep\I386\$OEM$ folder in the following folder:
   RemoteInstall\Setup\ Language\Images\ Riprep_dir_name\I386\Mirror1\UserData
   Note Language is "English" for the English language, and Riprep_dir_name is the unique name that you selected for the Riprep image.
2. Copy the 48bitlba.inf and Cmdlines.txt files into the $OEM$ folder.
3. Modify the Riprep.sif file (and any other template files for this Riprep image that you have created) in the following folder:
   RemoteInstall\Setup\ Language\Images\ Riprep_dir_name\I386\Templates\Riprep.sif
   To do this, add the OemPreinstall and InstallFilesPath values:
   
   [Unattended]
   OemPreinstall = "Yes"
   InstallFilesPath = "C:\sysprep\i386"
4. Close and then save the file.

OEMs can turn on this support by using the Microsoft Windows OEM Preinstallation Kit.

So I can't install XP to a SATA HD without doing all this? I have a dual-boot XP setup on my home computer, which is SATA, and I don't remember having any problems installing the second version of XP. I built that one from scratch, so I probably had the MB drivers at hand. Is it simply a matter of putting them on a CD and loading that via F6?

DBear13;

The process covered in detail in SpeedStep's post is the Microsoft instruction for using drives larger than 137 GB with the original release of Windows XP having no service packs.  I'm sure that this description was given because you stated that the Windows XP setup program reported only 137 GB of space on your hard drive.  The first version of Windows XP that could deal with hard drives larger than 137 GB was Windows XP including Service Pack 1.  The implication of your statement is that your Windows XP setup disk is the original release that does not include any service pack.

Another problem that you may have with your present Windows XP setup disk is that the Optiplex 755 uses a PCI Express video bus.  In order to have the driver for that video bus you must be using a Windows XP setup disk that includes Service Pack 2.

Lastly, the commercial versions of the Windows XP setup disk did not natively include drivers for a SATA controller.  In order to get the SATA drivers loaded when using such a disk it is necessary to use an internal 3.5" diskette drive.  There are only three types of USB drives that will be found by Windows XP setup, likely all obsolete by this time.  Check your System Setup to see if the BIOS will assign the USB drive to be drive A.  If it will you may be able to use it to load the drivers.  Otherwise you must use the ATA setting to load Windows.  After you install Windows using the ATA mode you can install the SATA drivers using the Update Driver feature of the Device Manager should you really want them.  One thing you should note; Dell used to ship Windows XP installation disks that had the SATA drivers included on the disk.  I know this to be true because my XPS 410 Windows XP reinstallation CD installs just fine using the RAID On setting, something that can't happen if the SATA drivers are absent.  You may have something similar with your home computer, but your disk will not necessarily work with your Optiplex.  This is because there are several different controller hubs used in Dell machines, and the drivers for your home computer may not be compatible with the hub in your Optiplex.