Here is what I have used for a good setup for a dual boot in my earlier linux/windows days
When you start up the setup program It will ask you to set some simple hardware, ex: mouse, keyboard, etc....
The first spot you probably have trouble with is the partition setup. I have never been able use the automatic partition tool to get dual boot to work. When it comes to that point you should manually partition the drive. Note: you need to have an unused partition or partition that you dont want to use any more. If you dont have a partition already, you should get a 3rd party software such as partition magic to make yourself one. There are other ways (fips for example.....but I have no idea how to use these
Once you have a decent amount of space set aside for the linux install you can enter the manual partition phase. You need 3 different partitons to run efficiently. If you have a Windows partition set up you will see it listed as either FAT32 or NTFS. Go to the unused partiton space (you will know b/c it says that it has free space to be used). From there you can click on the free area and create a new partition out of the unused space. You need to create these 3 partitions
/swap - About twice the amount of your RAM (not more than 512MB)
/boot - around 100MB usually
/ - (root partition) all the rest
These 3 partitions will get you up and running. You will want to set these 3 partitions as ext3 file systems (with the exception of the swap partition.....it has no filesystem). Once you have these set it will have you select the packages that you want installed and off you go. The GUI interface is designed pretty well. Dont be afraid to try things out, b/c until you actually go past the manual partition point, you havent actually dont anything to your hard drive and you can always make changes. When you are all said and done you will come up with a boot loader after it has you restart and you will be able to pick from either WindowsME (it will call it DOS by default) or linux. I use the grub boot loader and have never had any problems.
PS. Back up all your important info FIRST!!!!!!! I have used this many times and not had any problems but you never know