Here's how to stop squeaks in an old house with original floors you'd rather not replace. Preservationists use an acrylic injection technique that involves drilling holes through the subfloor from below, injecting a cushioning layer of water-based acrylic beneath the historic flooring. It's best left to a professional, but it does the trick.
For more on flooring, consider:
Wood Flooring 101
Fixing Squeaky Floors (VIDEO)
Old Wood Flooring: Replace or Refinish?