With Vista you should have had the opportunity to create yourself a set of Recovery Discs. I think it's a one-off option ie, once you've done it you can't do it again. Since I did it several years ago I can't check exactly how you can do it but, hopefully, I can point you in the right direction.
Go to "control panel > backup and restore centre". There you should be given the option to create a set of recovery discs. Follow the directions and you will end up with 2 or 3 DVDs.
Close down your PC, remove the system disc, replace it with your new, bigger one. Boot up your PC with recovery disc 1 in your DVD drive. Follow the instructions and you should end up with your new disc being the system disc and your computer should then be able to boot from it. You will then have to re-install all your personal programs.
If the process fails you've lost nothing, as you can put your old disc back in and you're back where you started. I did this a few years ago to increase the capacity of my system disc and it worked OK. Try it - you've nothing to lose except the cost of a few DVDs and some of your time.