I've found my XP PRO system blocked after the SP3 upgrade with the message :
"Windows Product Activation" "A problem is preventing Windows from accurately checking the license for this computer . Error Code : 0x80004005"

The computer is configured as follows :
PARTITION 1 in FAT32: Bootmagic with a Genuine XP SP2
(That partition is a fallback partition in case of troubles on the partition 2)

PARTITION 2 in NTFS : same Genuine XP SP2 upgraded (via download) to SP3

4 other DATA partitions in NTFS w/o any OS.

After the upgrade to SP3 the system was booting correctly on partition 2 NTFS.
To my surprise, my partition 1 FAT32 was found invalidated ????. No way to login except in Safe mode w/o network.
I did not understand why that partition was invalidated after that upgrade.

So I've recovered that partition from a backup dated of January 2008. (This is just a fallback partition, just in case so no mods since January)

Again a surprise, the newly recovered partition was usable, but now the NTFS partition 2 was invalidated.

I've booted on that partition 2 in Safe mode and here is the end of the MGDIAG
V 1.7.95 result :
(Genuine displayed in green with OGA status = 100)(note:MGADIAG 1.5 does not give that final part of the log file):

File Scan Data-->
File Mismatch: C:\WINDOWS\system32\winlogon.exe[5.1.2600.5512]
File Mismatch: C:\WINDOWS\system32\licdll.dll[5.1.2600.5512]
File Mismatch: C:\WINDOWS\system32\ntoskrnl.exe[5.1.2600.5512]
File Mismatch: C:\WINDOWS\system32\ntdll.dll[5.1.2600.2180]
File Mismatch: C:\WINDOWS\system32\kernel32.dll[5.1.2600.5512]
File Mismatch: C:\WINDOWS\system32\crypt32.dll[5.131.2600.5512]
File Mismatch: C:\WINDOWS\system32\advapi32.dll[5.1.2600.5512]
File Mismatch: C:\WINDOWS\system32\setupapi.dll[5.1.2600.5512]
File Mismatch: C:\WINDOWS\system32\oembios.bin[hr = 0x80070714]
File Mismatch: C:\WINDOWS\system32\oembios.dat[hr = 0x80070714]
File Mismatch: C:\WINDOWS\system32\oembios.sig[hr = 0x80070714]
File Mismatch: C:\WINDOWS\system32\syssetup.dll[5.1.2600.5512]

I've quickly found that in fact MGDIAG was fetching validation data from the FAT32 partition instead on the currently used NTFS partition.
BootMagic must start from a FAT32 partition, so I guess the Validation service is simply tracing the boot and wrongly gets the validation from another partition.
In order to assess that problem, I've rebooted in safe mode on partition 1 FAT32, and discovered that this fallback partition was not yet validated on line which confirmed what I suspected earlier.
The not yet valid XP partition 1 had invalidated XP on partition 2.

I've quickly revalidated that partition and .... partition 2 NTFS was revalidated too without running the online validation on that partition.

I've not yet checked what the validation process would return with Linux as a second OS in a similar dual boot configuration or with Lilo/Grub

Hope that info can help.