Here's neat tip I haven't seen before on PXE booting bare metal machine using the new Unknown Computer Support feature in ConfigMgr 2007 R2.  This one comes from Clifton Hughes in our Irving, Texas site:

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Issue:  Bare metal clients may be unable to PXE boot from a System Center Configuration Manager 2007 R2 Server when using the new Unknown Computer Support feature.  When booting a bare metal computer for the first time, it attempts to PXE Boot, and seems to try and load but then displays the following:

\smsboot\x64\abortpxe.com

After this the boot fails.

Cause:  This can occur if only All Unknown x86 Computers has been added to the Collection.  If it detects that the target computer is x64 capable it will not able to boot unless All Unknown x64 Computers is also added as a direct member of the Collection where you advertised your Operating System Install Task Sequence.

Resolution: Add both the All Unknown x86 Computers and All Unknown x64 Computers as members of the Collection where you advertise your Operating System Install Task Sequence.  Once you do this your new computer should successfully PXE boot and start downloading your Operating System Image.

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Thanks Clifton!

J.C. Hornbeck | Manageability Knowledge Engineer