Cannot update BootCamp Drivers 2.0 to 2.1

Question

My computer is a 24 inch White iMac (Late 2006 model), Intel based.
My current system is OS X 10.5.8
Basically, I recently used BootCamp 2.0 to partition my drive and install Windows XP Professional, 32 bit (x86).
I read that the latest Windows Service Pack my installation can contain is SP2, so I proceeded, and with many help from people in this forum, I eventually succeeded, managed to install the BootCamp Drivers and everything, practically everything except this and one game (which I suspect cannot run on SP2) works perfectly.
However, I want to upgrade to SP3, and to do so, I need to install the BootCamp Drivers 2.1, which are available from this link:
http://support.apple.com/kb/DL830
So I download the file, BCUpdateXP(.exe), but cannot run it, both on Windows and on disc when I burn the file to one.
Double clicking it does nothing.
So seeing it could be opened using Winrar, I did so, and saw two files: BCLocUpdateEnable.exe, which does nothing when I double click it, and BCUpdateXP.msp, which runs when double clicked, but quickly displays the message:
The upgrade patch cannot be installed by the windows installer service because the program to be upgraded maybe missing or the upgrade patch may update a different version of the program. verify that the program to be upgraded exist on your computer and that you have the correct upgrade patch.
The only option is to click OK.
Trying to install SP3 without this update just brings up a message saying I am 4 megabytes short to install it, even though I have 16.2GB free, and I have found that the message is caused because I do not have this 2.1 update.
My current Windows system is: Version 5.1 (Build 2600.xpspsp2gdr.100216-1441 : Service Pack 2).
The issue is posted by a lot of people, but none of them have solutions. And as far as I know, I should meet all the requirements to run this update; using SP2, BootCamp Drivers 2.0 installed.
It would be great if someone can guide me through this, I am sure it will help a lot of people with the same problem.
Many Thanks,
From Lelouch

Answer 1

Update on what I have tried:
I have tried changing the settings on location, language, keyboard etc as a post suggested. - No use
Tried double clicking the actual downloaded .exe file. - Nothing happens
I have tried extracting and clicking on the .msp file. - Get error message: The upgrade patch cannot be installed by the windows installer service because the program to be upgraded maybe missing or the upgrade patch may update a different version of the program. Verify that the program to be upgraded exist on your computer and that you have the correct upgrade patch.
I have tried doing what this post said:
http://support.apple.com/kb/ht1845
The command I have tried entering in the cmd.exe are:
msiexec /update C:\Documents and Settings\Lelouch\Desktop\BCUpdateXP\BCUpdateXP.msp
msiexec /update C:\Documents and Settings\Lelouch\Desktop\BCUpdateXP.exe\BCUpdateXP.msp
and both just bring up the Windows Installer window, and the button available to click on is OK, no Next as suggested by the help page.
Windows Installer only shows:
Windows ® Installer. V 4.5.6001.22159
msiexec /Option <Required Parameter> [Optional Parameter]
*Followed by a list of options.*
And the OK button.
I am tempted to try this:
1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey: HKEYLOCALMACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup
3. On the Edit menu, point to New, and click String Value.
4. In the text box under the Name column, type BootDir and press ENTER.
5. Right-click the name BootDir, and then click Modify.
6. In the Edit String Value dialog box, type the drive letter for your system drive, and then click OK. For example, if your system drive is C:, type C:\.
7. Close Registry Editor.
to have Windows SP3 installed, but not sure if it is safe as it does say to have BootCamp 2.1 installed first, but BootCamp 2.1 just won't install of all the things I have tried.
Anyone know if it's safe to do this?
Thanks if anyone can help me solve this.
Many Thanks,
From Lelouch