In addition to the other suggestions, it's not uncommon to have to run
a system recovery disk for windows to repair the windows boot sector
after installing Ubuntu in a dual boot setup. If you have run system
recovery in the past, you can usually access it without a disk during
the first few seconds after triggering a Windows boot by pressing one
of the upper row F keys. F2, I think. Generally, in sys recovery,
you simply have to select the partition Windows is installed to in
order to initiate repairs, but most recently, it generally just seems
to detects Windows installed on a partition and prompts you to repair
it..
After trying out the other advice, see the below links for more
information on using/obtaining a suitable recovery disk:
https://help.ubuntu.com/community/WindowsDualBoot
http://kb.wisc.edu/helpdesk/page.php?id=5097> You will want to select
the R option from the fist screen at this link. and disregard the
rest, initially. If that fails to get a working OS, move on with the
rest of the options in the link
Disclaimer- depending on where grub is (your linux bootloader) it is
always possible that you can damage that when using the window
recovery disk, now leaving you with a working Windows system, but a
non working linux install. I'll note that in the limited dual boot
installs I have had to repair windows on (about 5) I have not had this
happen, but if so, see the link below, or post another question to get
more help:
https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows