Personally, I haven't done a guided install in my life. I'm sure it's easier, but once you're stuck in your ways, sometimes it's hard to get out of that rutt.
I'd do this, but take this advice expecting things to be very different.
I go to manual install and set up my partitions manually. My manual partitioning guide would look like this...
60gb - NTFS
2gb - SWAP
16gb - /
385gb - /home
NTFS = Vista, for me.
SWAP = I'm sure you know, but HDD space allocated for RAM usage if RAM gets tied up.
/ = root (where the OS is installed)
/home = personal files... the "my documents" of Ubuntu.
Your objective here is to reinstall Ubuntu. Well, Ubuntu is installed on root. The symbol for root is /.
So if you split your home and root partitions, yours will look like what I typed above. However, otherwise you won't have a home partition... you'll just have a root partition.
*NOTE* - Your home directory is located within your root directory. However, when partitioning them you can split them accordingly to what setup you'd prefer. I split them so I can reinstall Ubuntu without it touching my music, pictures, etc... But if you keep everything as root (again, / ) then you will be formatting your home directory in the same shot.
But anyway, where YOU want to reinstall Ubuntu in the manual partitioning guide within the Live CD installation is in the / partition. Check that to format, mount it as root ( / ), and you'll be on your way.
AS ALWAYS - Back your stuff up before tinkering with anything, just as a general rule of thumb. I've learned the hard way many times...