same with me. I installed VB 1.6.2 recently and started a Windows XP image created with 1.5.6, as soon as Windows starts, CPU load reaches 100% forever.
Some users said this is due to crappy ACPI of windows, but I had activated ACPI support in 1.5.6 without any problems...
I'd be looking forward to a fix/workaround for this major issue...
The problem is not activating support in the VM manager -- you need to swap out the Windows kernel for one that doesn't use ACPI.
Your CPU will, by default, prompt Windows to load the ACPI-enabled SMP kernel. So when you switch to having Windows virtualized, this kernel remains and uses some ACPI registers that have severe virtualzation penalties (it talks to the BIOS). The solution is to force Windows to stop doing that. You can do this by going to Device Manager (right click on My Computer -> Hardware -> Device Manager), expanding 'Comptuer', right-clicking 'ACPI Multiprocessor PC', select 'Update Driver...', choose manual installation, from a list, and select 'Standard PC'.
Then you're set