Hello StuartLittle,
Here is the quick reason that you should go with single cpu and only do multi-cpu if absolutely necessary. It has to do with the way cpu's are scheduled in ESX.
Example:
You have two dual-core processors on your ESX host. That is 4-cores total. Life is good.
ESX needs to schedule a core to use for each vCPU that a virtual machine has. So, in a single-cpu VM it would schedule a single core to use when that VM needs to process. This leaves three other cores for the other VM's on the system to use. Yay!
What if the VM has two vCPU's? Well, ESX needs to schedule two cores simultaneously for that VM when it needs to process. So, when that VM has its processor time scheduled, the other VM's are left to fight over only two cores.
So, imagine if you start loading up your ESX server with multi-CPU VM's. They would constantly be fighting and waiting for processor time to be scheduled for them. This is where you will see high percentages of the %CPU RDY performance counter in VirtualCenter or esxtop on the host. %CPU RDY is the percentage of time that the VM is waiting for it's processing time to be scheduled.
So, the short answer is you CAN assign 2 vCPU's to a VM after conversion, but you probably don't want to unless it is absolutely necessary.
As for the HAL stuff. Yes, this is done in Windows Device Manager (Windows 2003 only. As far as I know 2000 you can't do this). Click the +sign next to "Computer" to expand it. If the driver is a multi-processor driver it will show up as "ACPI Multiprocessor PC" To change the driver back to a single processor, select "Update Driver". Select "Install from a Specific Location." Select "Don't Search. I will choose the driver to install." Then, select "Show All Compatible Hardware". Choose the appropriate driver. For a single processor VM this will be ACPI PC instead of ACPI Multiprocessor PC.
I hope this helps.
Don't forget to use the buttons on the side to award points if you found this useful (you'll get points too).
Regards,
Harley Stagner