das_stig :
http://support.asus.com/Download.aspx?SLanguage=en&m=P4P800+Deluxe&p=1&s=15
Select XP as O/S and then BIOS .. P4P800 Deluxe Beta BIOS 1021.006
Update shouldn't be needed to run a virtual machine of XP under 7, try using VirtualBox as your VM.
I can't update my Windows XPsp3 since 8/14 and without this upgrade my system will not be able to read WindowsUpdateDiagnostic.diagcab file.. Win'7 is the least OS to be able to read it. I've been through checking my system at:
http://forum.avast.com/index.php?topic=134766.0
Windows 7
Windows 7 Professional, Windows 7 Ultimate, or Windows 7 Enterprise.
Hard disk requirement: 2GB for installing Windows XP Mode. Additional 15GB of hard disk space for the virtual Windows environment.
Note: You can use Windows Virtual PC to run Windows XP Mode on your computer. Windows Virtual PC requires processor capable of hardware virtualization, with AMD-V™, Intel® VT or VIA® VT turned on in the BIOS.
http://www.microsoft.com/en-us/download/details.aspx?id=8002
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The diagcab file extention first appeared in Windows Vista and was used by Microsoft Support Diagnostic Tool (msdt.exe), so the earlier versions of Windows, e.g. Windows XP, has no ability to deal with .diagcab files. In Windows Vista, msdt.exe can run the script, but it requires a ticket from Microsoft support team to do the job so the use is very limited. Users of Windows 7, Windows 8 and Window RT will not feel any problem with .diagcab files.
http://www.diagcab.com/what-is-diagcab-file/