I've created my first stripped down game+internet XP image and appart from forgetting to insert my sound drivers I have lost hyper threading.

I need to do some research to tell me if I really want hyper threading on my game install as only one of my games specifically mentions it. But in the mean time. What setting did I mess up? and can in change the setting manually?

I remember a setting that seems likely from which I selected ACPI

That's the "Computer Type" (HAL) in Unattended part.
You can stil change the HAL in the Device Manager.

If it's not that then check chipset/mobo/cpu drivers, maybe you left out some that you shouldn't have. HyperThreading makes your CPU almost twice as fast, you don't want to lose it.

Hyperthreading requires one of the multi-processor HALs. It must be enabled in the BIOS.
-Normally the ACPI Multiprocessor PC is installed by default.

The performance enhancements are generally not spectacular: there is still just one core.
-Hyperthreading is not equivalent to dual-processor or, multi-core with Core multi-plexing.

So I am not sure that up to 2x improvement can ever be realized. If have read about 1.2x at best, depending on the main application.

I actually installed the processor a few weeks ago finally replacing my aging celeron and I had hyperthreading right away in XP and my stripped down XP and Ubuntu. It's not until I used nlite to create a bare bones XP that is so stripped down it'll do nothing more than play games and run a web browser (really really fast I should add) that I lost hyper threading in that install only.

After some more reading I've decided it's no real loss for playing games but I next want to make an ISO for my main install of windows with SP3 and drivers and license key. In that one I do run "normal" apps that will take advantage of HT so I need to nail down what I messed up to loose it.

Thanks for the suggestions and I will try HAL in the device manager but I would like to get it sorted all OOBE

There are several ways to utilize a multi core cpu efficiently. Speaking of own experience i usually set any games to go at core 1, leaving core 0 for system, as is default anyway. People have different way of doing this, you can either at some programs hex-modify the launcher, use a batch file with a utility that start the game/program exe at specified core only and with priority f.example.
I like Launchnset, its very simply, albeit a bit buggy on adding/removing things, but it works perfectly.
Tho it misses out on setting priority of launched game, it works enough and has nice small gui.
Performance wise, i'd go with a .batch file for each game, like cmd.exe /start game.exe /REALTIME, but u also need a thirdparty tool OR
modify game exe files, to launch it at only 1 core.
I know at least this is very poplular in the CPMA/Pro-mode arena, and possibly quake4 and similiar, not really into that new stuff myself.
Anyway, it will give you fps boost, unless the game you play is dualcore+ optimized, its the way to go.