Rarely you'll see something like:
So what does this all mean, and what is GRLDR?
Wubi uses GRUB4DOS combined with Grub2 to boot Ubuntu. GRUB4DOS has two parts: grldr.mbr and grldr. Wubi has renamed these to wubildr.mbr and wubildr (you are free to rename them as long as you insert the new name of grldr in the .mbr file).
The wubildr.mbr file corresponds to a grub legacy bootloader in the Master Boot Record. It is limited to scanning all partitions to find the rest of itself - the part that doesn't fit in the tiny MBR. These are the messages you see as it scans all partitions - and since it is based on grub legacy it outputs partitions with a zero index (unlike Grub2 which still uses zero based indexing on the drives, but not on partitions) i.e. hd0,0 in wubildr.mbr corresponds to hd0,1 in Grub2.
No matter where wubildr.mbr is, when it is called, it starts scanning the root on ALL partitions in the order dictated by BIOS to find the wubildr file. If it doesn't find it, you get an error that it can't find GRLDR, not WUBILDR. The reason it says GRLDR is likely a hardcoded message - for Wubi it's always WUBILDR that it can't find.
So what are the reasons for failure to find WUBILDR?
1. The rather obvious, but unlikely - there is no WUBILDR
2. Excessive fragmentation apparently
3. The wubildr file is > 137GB from the start of the partition
4. You have an ext4 partition that is read prior to the one containing WUBILDR.
WUBILDR.MBR cannot read ext4 partitions. The GRUB4DOS code it uses is from 2007 and it simply hangs when it encounters one of these. If you have an ext4 partition higher up in the BIOS order than your Windows partition, you're out of luck.
Some interesting points: when you install Wubi it installs WUBILDR.MBR and WUBILDR to the windows drive C:. But it also installs it to other partitions, like recovery partitions. If these happen to be physically before the windows partition, WUBILDR.MBR will find this WUBILDR and never use the one on your C: partition (it uses the first one it finds). And this is interesting as Grub2 updates the WUBILDR on your /host partition - not always the one used. Maybe this is a clue as to why Grub2 updates no longer boot on some installs.