You don't need to format windows or anything.
If you have a partition that you want to install the Ubuntu to, i.e. one thats not got any of the stuff you want to keep, then you should be able to install it direct to that partition only that partition gets formatted - the windows is left alone, unless you specifically tell the installer to "bin" it.
Obviously, when it comes to the bootloader part of the install you will need to put it on the first part of the hard drive so that it overwrites the windows bootloader (normal procedure), so that it can see all installed OS's. Yes you'd then have to change the config file so that windows is the default OS - thats quite a minor change.
Now if your hdd doesn't have any unallocated space/partitions you'd need to make some space. I understand that some distros can resize windows partitions (mandriva comes too mind), I've never had to do that as I used to use partition magic when I had windows installed - though I also have heard that PM has been known to cause a few problems when making linux partitions.
I understand that ranish partition manager
will do the trick (yes it's available too download as well).
The whole issue of partitions is quite thorny. Because different distros like to recommend slightly differing methods/versions/schemes as do more experienced users (my suggestions are always basic, because thats the level of my linux knowledge).
The suggestions vary from having seperate partitions for everything (that would be something like having /boot, /, /swap, /home, /var, /opt, /usr, /bin, /sbin, /tmp, /........... etc etc etc) In other words, vvv complicated with many decisions to be made for each one, right down to basically a single partition (actually thats 2, because just about all distros like at least a small amount of /swap space).
Lots of pros and cons of both. When I started with SuSE and Mandrake (now called Mandriva) both installed to a single partition with a bit of /swap - which worked fine until I got pissed off with loosing all my customisations, personal data, address books, etc etc when I wanted to try a new distro/version. I quickly learned that it's good wisdom to have a seperate /home partition. Everything you do is done from your home account, but if you want to install a different distro you just put it into the / partition (/ = root). Then as long as you install the same packages/software, things in the /home work fine (this technique should prevent one from making regular backups of course).
Since I arrived at Gentoo as my distro of choice, I use their default suggested partitioning scheme (/, /swap and /boot) with the addition of a /home. It works great and I haven't lost anything at all, despite having to learn how to mend a few minor disasters recently.
Emmanuel_uk's suggestion is also good (ha! mine is cheaper - no extra hardware to buy
). Lots use that idea. Personally, although I've toyed with the idea of the extra hardware (I should re-install windows for my partner, but I'm holding off the nagging with security concerns and "spoof" excuses), but have never found the need - and I can assure you that I'm no expert (I'm a professional trucker - not much use for PC's and networks etc on the road with 28 tonnes of whatever in the trailer
I'm guessing, but I'd think that Ubuntu would install to a basic partition scheme, so if you make some space and then boot the disc - you should be able to get it installed without any problems.
p.s. One further consideration, is that because of read/write issues from linux to windows and vice versa, lots of people also make an extra FAT32 partition, so that any data that you want to swap between OS's is accessible from both, and you can read/write too/from both.