Computer > C:/ ?
Your C:/ drive would be your Windows partition.If you used the Ubuntu Windows installer whatever that's called, it won't work for some reason.If you booted into the live cd and created a new partition and installed it there, it should work fine.
How do I access the Windows drives?The Windows partition where you installed Wubi is available as /host within Ubuntu (Places > Computer > File System > Host) All the other partitions will be available under Places > Removable Media