TWO STEPS FORWARD, ONE STEP BACK
To my amazement, nVidia Customer Care actually cared to respond to my question on this subject. The link in their response is a real gold mine of relevant info, and the LaptopVideo2go web site also very educational (see below).
Following the download and the instructions, I indeed managed to install the 270.61 video driver, and it seems to work - sort of. That's the two steps forward.
However, on reboot my User Account (and all related settings) were lost and replaced by the Administrator account. Secondly, in the System>Hardware listing it show a small yellow exclamation mark against the video card indicating some unspecified problem. I did notice that the driver shows GT525M instead of GT555M, if that makes any difference. Several Restore System ops eventually gave me back my user account, but on the next reboot it was lost again. That's the step back.
nVidia's response follows:
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I understand from your email that you are facing issues while trying to locate the drivers to support the GeForce GT 555 M on your Dell Laptop with the Windows XP 32-bit OS
The Laptop manufacturers purchase only the chipset from NVIDIA and custom design the graphic cards and the drivers to support customized features for specific Laptop models. The drivers available on the NVIDIA website may support only the basic features of the graphic chipset and may not enable customized features supported by that specific Laptop model
Please note that there are a few options available to update NVIDIA drivers for notebook computers
a) Primary Source = Notebook Original Equipment Manufacturer (OEM)
The primary source for graphics drivers is from your notebook OEM. These drivers have been fully qualified by the OEM to work with their specific notebook customizations. NVIDIA does not control the schedule for updates to these OEM drivers
b) Optional Source = Direct from NVIDIA
There are no graphic drivers from NVIDIA to support this graphics card with the Windows XP
c) Third Party sources
However if there are no available drivers with your notebook manufacturer or NVIDIA and you require a display driver, you may try a third party website like www.laptopvideo2go.com. This website hosts desktop display drivers which have been modified to install on a notebook
Please keep in mind that since these drivers do not contain any customizations for your notebook, they may not work as well as those provided by your laptop manufacturer nor does the maker of your laptop or NVIDIA support them
For your convenience, I am providing you the driver links to support this card while on the Windows XP 32-bit
http://www.laptopvideo2go.com/drivers/xp/270.61/disclaimer
Please click the link below for instructions on how to install these drivers (ForceWare Updater's Quickstart Guide, Want to update but don't know where to start?)
http://forums.laptopvideo2go.com/topic/33-have-disk-method-for-windows-xp/
Please let us know if the above steps have helped us to fix the issue
Warm Regards, NVIDIA Customer Care