It could be Wubi. It's worked fine for me on a couple of computers, but I have heard that sometimes it just doesn't seem to agree with a particular computer.
If you got from the Ubuntu website, that is the best place to get it from, since it's a Ubuntu creation.
I would recommend a visit to the Ubuntu Forums and searching for "wubi not working." The search will turn up several threads. http://ubuntuforums.org/
There's also a Wubi Guide, but I glanced at it and it doesn't seem to address your situation of being able to boot but not log in.