Look if you have a working Grub, which is Grub2 in Ubuntu, you can boot any operating system up "manually".
Have you supplied the information suggested by zirias
we would have been able to help you better.
It is possible that your Windows has been configured incorrectly and here is how you boot up the Windows manually
(1) In Ubuntu 10.04 select "Application", "acessories" and the click "terminal"
(2) Become the super user by this command in terminal
You need to answer it with your normal user password.
(3) Check the partition layout by the command suggested by zirias
copy and paste the command here if you are not sure the last character "l" is a digit one or a small "L".
(4) A Windows partition that boots must have a NTFS filing system so its partition ID must be 7. Also a MS Windows must be booted from an active primary partition which is always marked with a "*". From the "fdisk-l" output you should find which one it is. Say this is sda2, assuming you have a utility partition from the computer vendor and so your MS Windows has been placed in the second partition of the first hard disk. I am guessing as that must be a reason why Ubuntu got it wrong. This will be known to Grub2 as (hd0,2). Grub2 counts disks from 0 but partitions from 1.
(5) Reboot your PC but donot pick any operating system. Just press the "c" key to drop into a Grub prompt.
(6) Boot up your MS Windows manually by these Grub2 command and adjust for your situation is your Windows is not in sda2 or (hd0,2)
Let us know the progress so that we could tell you to configure the menu permanently.