The best thing you have to do first is to repair your boot sector.
Shortest way for you : Take your windows Installing CD and let it go
Until you have the possibility to type [R] (validate the License
acceptation if required). Then it may take several time, a part of
which with black screen. When you get again indications, turn
numerical keyboard zone on and enter the number of the proposed
partition (generally = 1) and you may also be asked for the
admin's password (if there is none just hit [Enter]. At that point
the rescue console is ready. Type "Fixmbr" and confirm, don't
care the message. Idem with "Fixboot" (no quote of course)
then enter Your Ubuntu Live CD and Type "exit"
Rebooting on the CD choose installation and once reached
the partitionning point choose "manual" option (easy)
You can the see your Windows partition, and you have to
make (if not yet done) 2 partitions : a little (2 Gb max) for the swap
and another one for Ubuntu (according the part you want to keep
for Windows and How you want for Ubuntu (40 Gb if you can is
not bad). Then you have to mount your partitions.
For Windows it's easy, just choose the format (ntfs) and take
a simple mount point /media/Win for example (so you'll be able
to see and to write on your Windows partition). For Ubuntu
choose ext2 if you don't need your sytem to be journalized
and root for mount point that is you just have to enter a slash (/)
Then go on and everything will go right.
When installed Ubuntu is naturally much faster booting than from a CD.
(let's say between 20 and 50 seconds after the sound driver as loaded,
then after the little noise in the speakers)