I have this Dell system, also upgraded to SP3.
What version of rdbss.sys do you currently have - check file properties, to see if it is a genuine Windows file
or a malware pest. The file is located in the folder C:\Windows\System32\drivers. Right click on it and select 'properties'.
You could run Malwarebytes and / or SuperAntiSpyware to check your system first.
Important: Rename the downloaded mbam-setup.exe file to mynewscanningfile.exe to help work
around certain malware that will block it from being run, or to a name of your choosing.
I suppose that if the only error relates to rdbss.sys, it would be worth replacing the file.
Whilst there are copies of the original file in the I386 folder on your computer and on your reinstallation disk,
as you have upgraded to SP3, you need an updated version.
(Current file on my computer is 172KB - 13.4.2008 v 5.1.2600.5512 (xpsp 080413-2111).
btw, if your system was bought around 2003, you should have a Maroon? Dell Disk, not a Windows XP CD,
with the following info on it:
' Operating System
MS Windows XP Home Edition
Including Service Pack 1 '