Sounds like you've got part (1) below, but maybe you don't have part (2).
(1) Are you sure you have a DSR (PC Restore) partition? When you boot into XP, take a look in Disk Management (Start>Run>"diskmgmt.msc") and look at the schematic "map" of Disk 0. If you see an "unknown" partition of about 3-5 GB at the end of the disk with no drive letter, that suggests your DSR partition is still there.
(2) Do you have a Dell MBR? This is easily overwritten, so even if your system shipped with the Dell MBR, it might not necessarily still have it. The telltale sign of the Dell MBR is a black screen with a single blue line at the top, with "www.dell.com" in white-on-blue letters. This screen should appear for 2 seconds between the POST phase (Power-On Self Test, with the giant blue Dell logo) and the XP splash screen (when XP starts to boot).
(3) If the answer is yes to both, then make sure you're pressing Ctrl+F11 at the right time. When the POST phase starts, you'll see the blue Dell logo, there will be a brief delay, then the "F2" and "F12" prompts will appear in the corner of the screen. After POST completes, you'll see the 2-second pause with the blue-line, followed by the XP splash screen.
You must press Ctrl+F11 after the F2/F12 prompts appear, and before the end of the 2-second blue-line pause. Hold down the Ctrl key, press down the F11 key, then release both keys together. (Note: contrary to urban legend, it's okay to press the keys more than once.)