First of all, make a backup of your Wubi installation file. That should be the file
C:\ubuntu\disks\root.disk. Do not try to boot your Wubi installation before doing this, as this may overwrite the data you deleted.
To try and recover the data, boot your computer from a Linux rescue CD (or copy the Wubi file to a Linux computer, or to a removable media).
Then use an undelete utility to try and save your data. For possible approaches, see e.g. How to recover a removed file under linux
If an undelete utility refuses to read a filesystem from a file, you can create a loopback device (as root):
# losetup /dev/loop0 /my/wubi/file
Afterwards, the file will be accessible as a device under
/dev/loop0, where you can use tools on it (or even mount it, though that will not help you).
To remove the loopback device when you're done:
# losetup -d /dev/loop0