newbie: help with disk partitions please

The WinDos boot partition is the one with 'ntldr', so you can't touch that one (although it's also always the one that has the WinDos OS too). During install you will have to change the primary partition type from ntfs to ext2 (or ext3) for Linux. You may consider deleting that partition and making 2 partitions (one for swap). You will lose all data on that 10.5GB ntfs partition when you do this.

Be extremely careful - back up all your valuable data (and pictures!) if you can, because one mistake can do a lot of damage. I'd suggest backing up the partition data, unfortunately the tool that can do it can just as easily wipe out your partition data and I think in all it only increases the chances of doing something horrible.

Anyway, back to an install - run the install disk and at some stage you will be asked to select a partition. At this point you should have an option for manual creation of the partitions and you should use that option to either change the primary partition's filesystem type or remove that partition and recreate it. Just changing the partition type should be the safest; if you delete/create a partition, the 'ntldr' may get confused (partition numbers effectively change) and you'll have to use some other tricks to get WinDos booting again.

After writing out the updated partition table, you should be asked to format the partition and then the ext2 (or 3) partition should be selected as the "root filesystem". At the end of the install, the installer may detect and set up the bootloader to 'chain load' to 'ntldr' (and boot WinDos), but if it doesn't do that, it's pretty easy to fix.

If you have a fast connection and a CD writer, I'd suggest making a "Live CD" which you can use to help fix things if something goes wrong. Occasionally the bootloader doesn't install properly and you end up with an unbootable system that needs a bit of fixing - in cases like that a "live CD" is far more convenient to use than the Install CD (although you can use the Install CD for the job).

Have fun. Read the messages on screen very carefully. If something scares you (as long as you haven't already written the partition tables) you can just quit the install (power button works) and ask some questions before trying again.