What installation disk are you using?  The Dimension 9100 has SATA hard drives which normally require an AHCI driver.  If an installation disk was shipped with the computer it may have this driver included, but if you are using a commercially purchased copy of Win XP Pro, that disk most likely does not.  If this is your problem, your choices are as follows:

1.  Restart the computer, press F2 during the self test to bring up the system setup page, navigate to the SATA controller menu, and set the controller to RAID Autodetect/ATA.  This mode emulates the older IDE/ATA controller, a driver for which is included on all the Win XP installation disks.  If you are not planning to use a RAID this is probably the most expedient solution.

2.  Download the SATA drivers from the Dell support page and expand them onto a 3.5" diskette.  Boot from the Windows XP CD and press F6 when prompted to begin the load procedure for third party drivers.  Insert the diskette when asked so that the SATA drivers will be loaded and setup can continue.  The problem with this approach is that an internal diskette drive is required in order to load the files from the diskette.  Windows XP setup will not recognize most USB diskette drives and insists that the files be loaded from the A: drive.

3.  Go here:  www.nliteos.com.  Learn how to slipstream the drivers into a do it yourself installation CD, make the CD, and use it to install Windows.

The second problem you may be having results from using a Windows XP installation disk that does not have a minimum of Service Pack 2 included in it.  The Dimension 9100 uses a PCI Express video bus for which earlier Windows installation disks have no driver.