Unable to complete the Installation of Service Pack 2 and Windows

I am an advanced PC user working with a customer's Dell Dimension 4500S with
Windows XP-Home Build 2600.

I am unable to download and install Service Pack 2. The download seems to
finish, however, the installation stops at the half way point and then stops
and uninstalls. (Tried six time from a sp-2 CD and direct downloads). Of
course, subsequent Windows upgrades do not download until SP-2 is in place.

I have read several KB articles to no avail. The one that seeems the most
promising is 873148, which seems to imply a registry key needs to be changed
(and I agree with that diagnosis).

Here is where I get stuck on KB 873148:

"Step 2: Locate the specific registry key

As soon as you have verified that a registry permission failure has
occurred, you must locate the correct registry key to change the permissions
and enable Windows XP SP2 to install successfully. To do this, follow these
steps:
a. Open the Setupapi.log file. By default, this file is located in the
C:\Windows folder. To open the Setupapi.log file, click Start, click Run,
type %windir%\setupapi.log, and then click OK.
b. Press CTRL+END to scroll to the end of the Setupapi.log file.
c. On the Edit menu, click Find.
d. In the Find what box, type Error 5: Access is denied under Direction,
click Up, and then click Find Next.<===== I CAN NOT FIND THAT ERROR TYPE SO
I CAN'T GET TO THE REGISTRY KEY TO CORRECT PER STEP THREE.

The found entry in the Setupapi.log file should be similar to the following
example:
#-007 Deleting registry key HKCR\vnd.ms.radio
#E033 Error 5: Access is denied."

Will some smart person get me unstuck. Thanks in advance.

I should add that I disabled the AVG antivirus software and there is no
firewall.

I removed all the viruses and spyware.

Zonny