Yes, we purposely don't display the CPU meter under Windows XP due to a bug in some plugins.
Which plugins are you using on that track?
Try mixing down to a wav file with the same sample rate, and if the problem happens again, quit Mixcraft and send us your mixcraft-log.txt file.
To find the mixcraft-log file, highlight the following line and press Ctrl+C to copy it:
Press the Windows Key+R to pop up the Run box in Windows. Click in the Run box and press Ctrl+V to paste in the line from above. Finally, click OK to go to the location of the mixcraft-log.txt file. Submit an Acoustica Support Request
and attach the log file along with a brief description of the problem.