Alright, if your product ID is XXXXX-640-0000356-23XXX and XXXXX-640-2001765-23XXX, Microsoft will not allow you to install Service Pack 1 due to the fact that a large number of those serials were issued to corporate customers, and 90% of pirated XP copies out there are installed using those serial numbers. However, there are legal copies out there, and Microsoft will help you out.
First you need to change the installed serial number. Make sure you got a valid serial number from Microsoft. Backup the registry and:
Log in as the local Administrator
Click Start > Run > and type in Regedit
Browse to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\wpaevents
In the right pane, right-click OOBETimer, and then click Modify
Change at least one digit of this value to deactivate Windows
Click OK and close regedit
Click Start > Run and type in: "%systemroot%\system32\oobe\msoobe.exe /a"
Click Yes, I want to telephone a customer service representative to activate Windows, and then click Next
Click Change Product Key (at the bottom)
Enter your valid Corporate Product Key
Press Update and close the window.
If you are returned to the previous window, click Remind me later
Restart your computer
That should fix it. Now you can call Microsoft and ask them for a serial number if you don't have another one.
