I had this exact same problem with Ext2 IFS on XP, maybe it's the same thing on Vista...
From http://www.fs-driver.org/troubleshoot.html I downloaded the moundiag.exe and ran it
against the drive (G:) that was showing up in explorer. This gave me an informative error (sorry I don't have the exact text) that indicates that it won't work with inode size = 256 which is what my Linux system created them with by default.
So... I hooked the drive back up to my Linux system, made sure I had a good backup, and then used fdisk to delete the existing partition [AND ALL EXISTING DATA] and create a new one -- then did a:
$ sudo mke2fs -t ext3 -I 128 /dev/sdg1
Once that was done, I restored the data, unmounted it from my Linux system, and was able to connect to it with my XP machine (and Mac OS X, too -- using Ext2FSX).
Also noteworthy is that Ext2IFS can ONLY use the first partition... quoting from the troubleshooting guide (link above):
So there is one straight advice only: do not use removable media with more than one partition with Windows.