Did you ever figure this out? No one from McAfee is likely to help you as you are breaking all the rules here: Unsupported OS, competitor's AV, not providing version details etc...
You may be out of luck here. If you enabled Solidcore on boot and didn't configure the necesary updaters for Windows XP and Forefront, it's probably your culprit.
Next time, make sure you configure Solidcore to be in Observe mode and watch those events to confirm (Disabled Mode is also an option).
Can you boot Safe Mode? Last Known Good? A PE environment using CD/DVD, PXE? If you can get into the OS any of these ways, you can open up a command prompt and access the CLI where you can disable Solidcroe and try to boot again to confirm... (sadmin status, verify status, sadmin recover, enter password, sadmin disable, sadmin lockdown)