I tried installing Ubuntu 12.04 using Wubi and it just doesn't work, every time, without fail. I had the following scenarios:

1. I downloaded only wubi.exe and ran it.  
The wubi installer started downloading the amd64.iso using torrent. But when there were just about 40 secs left to download, it shows an error 404: File not found.    

2. I downloaded the iso file seperately and put it in the same folder as the wubi.exe. Now there are two cases:  

  a. Offline: wubi says it could not download the metalinks file and hence cannot download the iso.
So I download the meta files separately and place them in the same directory. wubi shows the same error again. 

  b. Online: wubi works in same way as in case 1. and the same problem occurs as in case 1.  
In short wubi doesn't recognize the already downloaded iso in the directory at all.  

3. I burn the iso into a Cd and run it. The same thing occurs as in Case 2.

Just in case that you know, I installed SP3 for win XP just before using wubi. While Windows is running alright, is it possible that its causing conflicts for wubi?