In my experience the Ubuntu installer
will give you the option on install to use Grub / a boot loader. The installer can definitely manage the installation and repartition your HD if there is space.
Personally I like to keep my installations separate (its a work laptop) so I always buy a cheap HD caddy from ebay and put a disk drive in it and install as a secondary drive replacing the optical drive and install to this (if your laptop has this option). I then use the laptop BIOS to boot to whichever I want (on a Dell this is F12 key on boot) and that way I keep my Windows disk completely separate and Linux mostly invisible to the IT dept.
If you do end up installing it on your main drive you can alter the default OS which GRUB2
boots to by editing the config file.
Ubuntu will also run quite well from a USB stick if you are just fiddling.
EDIT : I just noticed roseway's beautiful Sticky posts on Linux, installing and everything - worth a look