When you try to boot a Wubi install and you end up at a grub> prompt, then most likely it's the result of corruption. Wubi installs Ubuntu on a large virtual partition that is a single file called the root.disk (if you install on a FAT32 partition, other .disk files are created as well).
To confirm it you can run the bootinfoscript and you'll see something like:
sda1: __________________________________________________________________________
File system: ntfs
Boot sector type: Windows Vista/7: NTFS
Boot sector info: No errors found in the Boot Parameter Block.
Operating System: Windows 7
Boot files: /bootmgr /Boot/BCD /Windows/System32/winload.exe
/ntldr /NTDETECT.COM /wubildr /ubuntu/winboot/wubildr
/wubildr.mbr /ubuntu/winboot/wubildr.mbr
/ubuntu/disks/swap.disk
You can see it lists /ubuntu/disks/swap.disk, but it's also supposed to show /ubuntu/disks/root.disk.
If this happens, the first thing to do is to boot Windows and run chkdsk /f or chkdsk /r (the latter also looks for damaged disk sectors). This will repair any NTFS file system corruption and sometimes fix the damaged root.disk file (but not always).
Here's how to do it from Windows 7. Go to Computer, right click on the "drive" you selected when you installed Wubi e.g. C:\, and select Properties, Tools, Error-checking Check now, and then Start:

If you installed Ubuntu on the C:\ "drive", then you have to reboot Windows to let chkdsk complete. Make sure you don't hit any key at bootup or it will cancel the scan.
Now comes the tricky part. Sometimes Windows will remove the repaired file to a hidden folder called \found.000 or \found.001 (it increments). And in most cases the file name will be lost, so you'll see a file called file0000.chk inside there. In this case you want to look for a file that's about the size of the root.disk (at least 5GB and up to 30GB). You can move that back to \ubuntu\disks\root.disk and you should be good to go. If the entire \ubuntu\disks directory is repaired, then instead you might see a directory \found.000\dir0000.chk and inside that you may find root.disk and swap.disk. In this case, move the directory back, renaming it to disks.
Here is how to do it... hit the Windows key + R, type cmd, look above and right-click on CMD.EXE, and select Run as administrator. Then from the command like:
C:\>cd \found.000
C:\found.000>dir
Volume in drive C is OS
Volume Serial Number is B4B7-99A8
Directory of C:\found.000
19/07/2011 02:02 PM 15,000,000,000 file0000.chk
1 File(s) 8 bytes
0 Dir(s) 222,258,069,504 bytes free
C:\found.000>move file0000.chk \ubuntu\disks\root.disk
1 file(s) moved.
or if the whole disks folder is recovered
C:\found.000>move dir0000.chk \ubuntu\disks
1 dir(s) moved.