If you're talking about a situation involving networked Linux and Win boxes, then Samba is the answer. If your Win and Linux Partitions are on the same system, you might want to check out
explore2fs.
I haven't used it myself, so I can't vouche for its functionality (which appears to be "read-only" at this point), but it apparently lets you have some access to ext2 (not sure about ext3) partitions under Win.