If you need to open drives (for example: an USB-harddrive) formatted with the ext2/ext3 file system, you can use a freeware tool Ext2Fsd from the Ext2Fsd Project.

About Ext2Fsd:
Ext2Fsd is an ext2 file system driver for Windows 2000,  XP, Vista. It’s a free software and everyone can distribute and modify it under GPL2.

Features Ext2Fsd supports:

1. ext2/ext3 volume reading & writing
2. ext3 journal replay when mounting
3. various codepage: utf8, cp936, cp950 …
4. mountpoint automatical assignment
5. large inode size: 128, 256, …
6. large file size bigger than 4G
7. OS: 2k, xp, vista, server 2008 (i386/amd64)

Link:
http://www.ext2fsd.com.