How do you know which partition has Windows on in the Ubuntu installation screen?
That depends on how you've partitioned the drive.
There is a tool you can use when you use the Live CD ( try ubuntu without changing you hard drive setup ) call Gparted (it can be found under System->Adminastrator as Partition Editor).
This tool will allow you to create/delete/resize a partition as well as display file system information.
Unix/Linux will display the avalible drives as /dev/sda /dev/sdb (for SATA drives) or /dev/hda /dev/hdb for PATA Drives and a partition on those drives as a number (e.g /dev/sda1 /dev/sda2).
Windows file systems would be listed as ntfs (Windows 2K/XP/VISTA/SERVER 200x or fat32 (windows 95/98).
Be warned before you do any kind of messing with the partition table. Make sure you have...
1) An original manufatures disk (or one that can be created from the back up software supplied with the computer as a windows backup soultion is no good at all)
2) You back up any data you wish to keep.