If you really want to dual boot Linux then I'd consider first trying the
wubi installer.
This installs Ubuntu as a windows application and so can be uninstalled just as any other windows app. It doesn't partition your disk, but it does give you dual boot, so once you've booted into Ubuntu it's no different to Ubuntu installed any other way. The advantage is that if you find you don't like/use/want it, it's easy to get rid of. OTOH, If you decide to convert, you can still partition your disk or even wipe of Vista.
However, if you're just after a way to speed boot times (as most of the replies to your post seem to think) then have you tried running msconfig and disabling a load of the junk that starts at boot time ? Laptops are notoriously bad for this (especially Asus machines) as they come with loads of stuff pre-installed and by the time you've added Java, iTunes, quicktime, flash, realplayer, steam, Adobe Reader and a load of anti-virus / anti-spyware apps, etc. Startup times will become SSSssssllllooooowwwwwwww