Thanks everybody, that's it! NT returns the free space modulo 4GB, which is an unsigned 32-bit integer. So this problem can occur anytime there's > 4GB of free space. Nothing important about 32GB as I had first expected. Glad to know this is not a LabVIEW installer problem. The windows calculator exhibits the same thing. (Enter a number > 4GB, convert to hex and back to dec and you get a number modulo 4GB.)
Our appl needs 63MB to install, so this error will occur 63MB/4GB * 100 = 1.5% of the time on systems with large hard drives. Our "fix" will be to tell the customer to try the install, and if they get the error message copy ~70MB of files to guarantee that the resulting truncation won't be in that 1.5% window. Windows 2000 does not exhibit this problem. It uses a 64-bit word, as noted by the QWORD (Quadword) indicator on the calculator.
