Ext2 or Ext3 file systems are commonly used by the Linux operating system. Support for these file systems is not by default built in to windows operating systems.
I remember trying to access windows file systems (FAT and NTFS) from linux and linux file systems (ext2 and ext3) from windows while I was in college. Back then, there wasn't a stable driver for NTFS on linux. Drivers could read from NTFS partitions but weren't stable enought for writing to it. But now, there are drivers that can read/write to both FAT/NTFS and are pretty stable. And you can find these drivers included in linux distributions by default.
I was surprised to see the windows drives mounted automatically on ubuntu and delighted to find out that I could write to those drives. This solved half my problems of sharing data between my two operating systems - windows vista and ubuntu linux. I was further delighted after seeing the search results for "accessing ext3 from windows". I found three results useful. The first one was for Explore2fs - a GUI tool for accessing ext2/ext3 file systems from windows. I had already used this while I was in college and know it's limitations. Windows programs cannot access those file systems, and files had to be copied to a windows drive to be accessed from programs. The other two results (Ext2 file system drivers) were of much interest to me this time. I chose Stephan Schreiber's implementation and installed it on vista. This implementation comes with an installer. Can be uninstalled from Add/Remove programs. Provides the ability to mount the file systems in read only mode along with support for large files (files larger than 2GB). My linux partition was mounted as a new drive and I could access it from all windows programs just as other windows drives. Now, I no longer have to worry about where to put my data so that it can be easily accessed from both the operating systems.
FYI, the ext2 file system driver I didn't mention above was Ext2Fsd, an open source project available on sourgeforge.