Hi Brett,
According to PowerPoint's help:
"If you open a presentation that uses fonts that aren't installed on your
computer, PowerPoint will use the Arial font as a substitute. The original
font information is retained in the presentation, so the presentation should
print properly from another computer that has the correct fonts installed.
Try changing the fonts in your presentation or install the missing fonts
used by the original presentation."
Which doesn't explain why your fonts are being changed to Times New Roman.
Have you installed the updates for Office?