This
Ext2 IFS driver for Windows might help as it allows you to manage ext3 files from explorer. Check it out.
I personally have never used it because I'd rather Linux access Windows files and not the other way around, but I heard this is good software. Just keep in mind that Windows can't modify access rights properly, so actually it may be no help for you.
What would probably be more useful is to use something like VMware and set up a virtual machine on your USB stick. The trick to getting this to work the way you want it to, however, is to have the Virtual Machine use the actual hardrive and
not a virtual one. Then you can run the Linux OS through VMware, but you can also modify the ext2/3 files on the USB stick using the driver linked above (which might be better done through something like Cygwin, however).
Just keep in mind that you will need to dedicate the entire USB stick to do this.