Wubi install: choosing Windows at boot menu does not work

I'm not very good with computers. The thing is that I had a dual boot (win 7 and ubuntu 11. something) installed with Wubi for a long time and it worked perfectly. Now the problem is that I updated ubuntu to 12.04 LTS and I can only work with ubuntu. When restarting the computer I can see the boot options (win 7 and ubuntu) but if I choose to boot win 7, it begins to load and then it restarts. What can I do? I use a Dell 1520 vostro. Thanks in advance.

If you are dual-booting and NOT using Wubi:

Thats what I got once into. I too had the same problem you are having. Its nothing but a damaged MBR. You just need to do two things:
1. Repair the MBR
2. Re-install the grub.

For step 1, visit here and see how to repair MBR as explained in the best way I could.You will then get your window working and you will not be asked any operating system to boot into at the startup. Now next step (re-installing the GRUB) is to do that. You need to re-install the GRUB to have the same choice of the operating system. Just follow the next explained step.

STEP2: Boot into ubuntu with any live cd, and just go to the terminal and type following:

$sudo fdisk -l
.....list of /dev/sda
Note down the device name containing the root partition of your installed ubuntu, in my case it was /dev/sda2
......
$sudo mkdir /mnt/myfolder
$sudo mount /dev/sda2 /mnt/myfolder
$sudo grub-install --root-directory /mnt/myfolder /dev/sda

And you are done. Just reboot your machine and problem will be gone.