For reading the Linux Extended filesystems (ext2, ext3, or ext4) from Windows, there is the
EXT2READ application. There are some other utilities which permit Windows to access ext2 and most ext3 partitions seamlessly (that is to say, any file open/save dialog will treat the Extended filesystem just like a Windows partition); however, to my knowledge none of these yet support the ext4 filesystem.
Linux-based distributions are readily able to mount a Windows filesystem (FAT16, VFAT, or NTFS) by adding an entry to your /etc/fstab file. Most distros will offer to do this automatically during installation; if not, doing so post-install is not that difficult and I should be glad to instruct if provided details on your setup.
As others have suggested, having a third, intermediate partition (which is basically what your USB pendrive is providing) will ensure that you don't accidentally corrupt the other system's files but as long as you pay attention to what you are doing, such is not necessary (especially if you stick to read-only access of the other system).