Your question seems a little unclear to me. What is showing your images as TIF images? Is it Windows Explorer, or is it some other program?
DNG was shown as TIF by Lightroom 3.4 as well as by Windows Explorer, both when running under Windows XP SP3. However, Lightroom running under the same O/S has no trouble with the RW2 format.
If I go to Lightroom 3.4 running under Windows 7, the DNG is shown correctly.
That is really a odd behavior. I have run Lightroom under both Windows XP and Windows 7, and I have never experienced that. Only difference perhaps is that I'm running 32 bit, but that shouldn't make a difference in this case. Is there any chance that your images were converted on the XP computer?
As you may realize, I have no real answer for you. However, if others see some activity on this thread perhaps they will offer some further suggestions.
See >> below.