Honestly I would set up the partitions beforehand as repartitioning a live drive ends in tear more often than not in my experience. I'd also recommend Windows 7 over Vista but whatever, we'll roll with it.
I'm a bit perplexed to hear the installation didn't give you the option to configure partitions before installing. Usually that pops up. Is this an actual Vista disc or one of those build discs that come with a laptop? If it's the latter I don't know if you'll be able to do it during the install. Your GeekSquad person is right though and diskmgmt is an option once everything is installed.
Gah, I'm rambling again. Bear with me.
Ideal setup is for you to partition your harddrive before you even install. To do this you'll need a boot disk with some tools on it that let you fiddle with the partitions. I'd recommend burning yourself a copy of Hiren's Boot Disk, as it's pretty handy for all sorts of computer troubleshooting. Alternatively I think there are a few Linux options that have partition tools. Knoppix and Ubuntu come to mind. Anyway, once you have your bootable CD, pop it into your computer and boot up. You should get a command line screen with tools on HBD, or if you chose a Linux flavor it'll give you the option to run as a live CD, which basically puts together a mini-OS session on your RAM. Find your way to some partition tools and start by wiping out all the partitions. After that's done create a primary partition that will hold your OS. Make sure it's big enough to install the OS and whatever else you plan to stick on there and then add some wiggle room for all the updates Windows likes to download. This partition should use the file system NTFS, which is the standard file system for windows these days. Create a second partition, NTFS should work fine for this as well. This second partition should use up the rest of your harddrive space, otherwise you'll have a bunch of unused drive space just hanging out. I think you can also label the drives using some of the partition tools available, but don't quote me on that.
Now that your partitions are set up take out the CD and turn off the computer.
Next, pop in your Windows install CD and boot up. Hopefully you'll get an option for partition selection, or barring that it will at least automatically choose the primary partition you created earlier for your OS and dump the install there. You should probably also be aware that the drive you install your OS on is the one Windows will try to install all your programs and games on by default too. You don't have to install there, but it will go there first if you don't correct it every time.
If your Windows installation is being dumb and wiping your drive every time for some reason, you'll probably have to use diskmgmt like the guy told you. It's not so bad and even if you botch it you're not really losing anything with a fresh install. To bring up the tool go to the start menu and hit select "run"(alternatively: windows key + R). This will bring up a dialog box. Type "cmd" into it and hit OK. Now you'll be looking at a command line window. Type "diskmgmt" and hit enter. Assuming that Vista doesn't spaz out about admin rights or some such, this will bring up the partition tool which you can use to edit partitions.
I don't know my way around diskmgmt too well, but you'll probably have to shrink the partition that's taking up the whole drive first and then carve out a second partition from the leftover space.
That's about all I got.
