Assuming it really is SP3 for Windows XP and *not* something else...
Verify what specific version of Windows XP you have first...
Start button --> RUN
(no "RUN"? Press the "Windows Key" + R on your keyboard)
--> type in:
winver
--> Click OK.
The picture at the top of the window that opens will give you the general
(Operating System name) while the line starting with the word "version" will
give you the rest of the story. Post _both_ in response to this message
verbatim. ;-)
Do you have an HP branded computer with an AMD processor?
(If so - answer and wait. If not, answer and continue with the steps.)
Once you have provided that information, I suggest a little cleanup...
Do all of this in the order given. Do not skip a step because you
think/know you have done it before.
If you experience any errors, get any strange messages, want further
explanation for any step - come back. Post messages word-for-word.
Describe exactly what you see around the message and what you did right
before that may have caused the message.
Download, install, run, update and perform a full scan (separately) with the
following two applications (freeware versions are the ones to use for this):
SuperAntiSpyware
http://www.superantispyware.com/
MalwareBytes
http://www.malwarebytes.com/
After performing a full scan with one and then the other and removing
whatever they both find completely, you may uninstall these products,
if you wish.
Download and run the MSRT manually:
http://www.microsoft.com/security/malwareremove/default.mspx
Reboot.
Search your registry for %fystem and replace the "f" with an "s". May be
three or four matches, may be none. You may even have to take ownership
of the keys in order to make the change.
Then...
Ignore the title and follow the sub-section under "Advanced Troubleshooting"
titled, "Method 1: Reset the registry and the file permissions"
http://support.microsoft.com/kb/949377
*will take time
(** Ignore the last step - you will install SP3 - but not now.)
Reboot.
Download/install this:
http://support.microsoft.com/kb/290301
After installing, do the following:
Start button --> RUN --> type in:
--> Click OK.
(The quotation marks and percentage signs and spacing should be exact.)
Reboot.
Download/Install the latest Windows Installer (for your OS):
( Windows XP 32-bit : WindowsXP-KB942288-v3-x86.exe )
http://www.microsoft.com/downloadS/details.aspx?familyid=5A58B56F-60B6-4412-95B9-54D056D6F9F4&displaylang=en
Reboot.
and...
Download the latest version of the Windows Update agent from here (x86):
http://go.microsoft.com/fwlink/?LinkID=91237
... and save it to the root of your C:\ drive. After saving it to the root
of the C:\ drive, do the following:
Close all Internet Explorer windows and other applications.
Start button --> RUN and type in:
%SystemDrive%\windowsupdateagent30-x86.exe /WUFORCE
--> Click OK.
(If asked, select "Run.) --> Click on NEXT --> Select "I agree" and click on
NEXT --> When it finishes installing, click on "Finish"...