We're in the process of re-imaging our school lab computers - HP dc5000.
We updated the WinXP image with Windows update and virus software.
Now, after installing the image on approximately 90 computers, 5 are
reporting that Windows needs to be activated due to significant hardware

We are using a volume-license install of WinXP. We have been able to
activate a few over the Internet, but some will not due to the key being
used too many times.

The only hardware difference is some machines have 512MB memory while
others have 1GB. The image was created on a 512MB machine.

What can I look for?