Installing Windows after Linux is more difficult than the other way around, especially for an inexperienced user, the simplified order of events is for you to partition the disk to make room for Windows but making sure you partition the disk as NTFS, this can be all done in GParted which comes with Ubuntu(I think, if not then run "sudo apt-get install gparted" from the command line) when you're installing Windows you then use the free partition you created and it will install but it will over write your mbr and remove grub which is the Linux bootloader. You then need boot into a Linux live cd(any cd that supports live booting, would recommend Ubuntu as you're probably familiar with that) then repair grup which is a few commands and when you boot up again you'll have the same boot menu.
The easier option is to boot into a Linux live cd and reformat the disk to NTFS and install Windows and then Ubuntu, would be much easier for a new user. But this assumes that you're ok with reinstalling Ubuntu, that it doesn't have configuration or data you need. Depends on your scenario I guess.