Well, I looked at that site, but was put off trying anything because it was so poorly written and it wasn't clear whether the 'solutions' offered were options or had to be completed in sequence.

I did remove Spybot Search and Destroy whilst I tried again - but it made no difference. In any case, it hadn't affected the other machine.

As for 'running as an admin' - what on Earth does that mean?

Running as Admin is the type of user account you use. An Administrator account has pretty much global permissions over the computer. A limited user account has much reduced permissions, and generally can not install nor uninstall nor update most software that affect more than that account.
Go to Control Pnael>User Accounts and look at the account you use. (Your logon.) It will say whether it is an admin account or not.
The options on the site I linked are generally intended to be sequential. If the first one doesn't solve the problem, try the second. If that doesn't help (and you still want it fixed badly enough) try the third, if you're up for it.
Think about what is different between the two machines. Do they both have the same settings, firewall, and antivirus? Is anything running on the primary that isn't present on the backup?

Both accounts are identified as 'computer administrator'.

The only difference I can quickly establish is that the AntiVirus 'Enable Script Blocking' is ON for the computer with the 'Access Denied' issue, whereas the AntiVirus version on the other computer has no such option?

I tried installing with the 'Autoprotect' disabled, but still got the 'Access Denied' warning. Presumably that made no difference because nothing was being actively downloaded from the Internet (I'd unplugged the connection to the router).

Would selecting 'Enable Script Blocking' OFF be likely to make any difference?

I wouldn't think "enable script blocking" should make any difference to a program update, that's more to prevent dodgy items in hijacked or dodgy web pages from running.
What AV is it? And is it the same AV on both?
Ditto question the firewall.
Ditto question any other application eg antispyware/behaviour blocker etc.

I have to tell you I'm not an expert, but have troubleshot a few of this type of issue.
Is the primary, perchance, an HP or Compaq presario with AMD processor?

BEagle try reading this Microsoft support document: http://support.microsoft.com/kb/949377

You may need to dive into some of the links in it as well.

Well, a couple of months later and SP3 downloaded itself automatically, but still came up with 'Access Denied' when I tried to install it.

The Microsoft support site just sent me round in circles, so I gave up and rang them.

2.5 hours on the phone with a very patient and helpful chap and we found the problem - 'registry permissions', whatever those are. Plus I needed to free up some space on the hardrive - although that hadn't been an issue before.

All then installed and re-booted fine. Thanks, Manish!

So, 3 months on from first post - I am still 'holding back. :) What's the verdict, guys? Any probs generated?

It's as it was then... SP3 is mainly a rollup of the fixes released post-SP2. If you're fully patched there's hardly any difference. There's a few things under the hood to facilitate Windows Server 2008 (yuck!) connectivity, but not much to write home about.

As Windows service packs go, it's rather uneventful.

True, but when I asked what was the point of installing SP3 when I was already up to date with all other downloads, the Microsoft chap told me that many new programs, such as subscription Antivirus updates, will require you to have XP SP3 as the minimum OS.

I follow a broadcasting Web group here in the USA. One member, who is the IT person for a cluster of stations, says that some of the software they require will NOT run under SP3. I haven't dared try it on "mission critical" computers. I think one should be prepared to uninstall SP3 if one chooses to try it.

SP3 will not allow the address bar to be put into the taskbar which is a real inconvenience to some of us.

I am back on SP2

MC

XP3 Update causing wireless netwok probs - Older Kit.
 

I have fould that XP SP3 upgrade kills the inbuilt wireless on my AJP 900 Series notebook clone. It installs just fine; but WPA connectivity is lost (I'm using WPA1 - I.E the TKIP variation - because I have older kit sharing my network) - Microsoft have released a Registery Fix to prevent XP3 form automatically installing - Its here Disable XP SP 3 and Vista SP1 From Windows Automatic Updates » Raymond.CC Blog - or google XP Blocking Tool - Its reverseable too.