A link to the infamous "Dual Core Hotfix" (aka KB896256) everyone is referring to, with some background info
"...Because Windows XP was not originally designed to support performance states on multiprocessor configurations, changes are required to correctly realize this support on multiprocessor systems. Windows XP Service Pack 2 includes the required changes to the kernel power manager. These changes make sure that Windows XP correctly functions on multiprocessor systems with processor performance states.
This hotfix also addresses the following issues on computers that have multiple processors that support processor performance states:
• A possible decrease in performance on single-threaded workloads when processor performance states are using demand-based switching.
• The synchronization of the processor Time Stamp Counter (TSC) registers across processors when you use the ACPI Power Management timer on multiprocessor systems.
• ACPI C-state promotion and demotion issues in the kernel power manager..."
http://support.microsoft.com/kb/896256
File name: WindowsXP-KB896256-v4-x86-ENU.exe
Solved my problem!
DIGI you really should post this somewhere on your site that's easy to find. This is critical to Pro Tools operating as promised on a Windows XP/Multi Core system...