If the usb drive is formatted with an ext2 or ext3 file system, you would need something like "IFS Drives" (See http://www.fs-driver.org/
I just plugged an 8GB sb drive formatted with ext2 into an XP system with "IFS Drives" installed, and it mounted just fine. It says that it works on Vista, but I don't have Vista. So, YMMV.
It has a bunch of restrictions -- like it doesn't understand LVM, for one -- but it is a nice piece of code, and freeware.
(I've made my donation, as it works nicely for me.)