What I did (the third time round
) - edited to help you:
I downloaded Ubuntu 10.10 iso and burned it to a DVD-R
I booted from CD (F12 in the BIOS) and entered "Try Ubuntu"
I used GParted (topbar:system>administrative) to shrink my Windows 7 partition by 30GB
I then made an Extended partition, and made a 5GB Swap Partition and 10GB & 15GB EXT3 partitions
I then clicked on install Ubuntu (although I would advise you to leave the CD and boot up W7 to check its OK)
In that I went into Custom partitioning/installation (sth like that) and clicked on the 5GB Swap and maked for Swap, clicked on the 10GB EXT3 and used for "/" and clicked on 15GB EXT3 and used for "/Home"
There is a thing at the bottom for bootloader installation - want to keep W7 as your main? Then make sure GRUB (the bootloader) is installed on the 10GB EXT3
When you reboot you wont see any change - Windows should boot as usual
In Windows, google and install "EasyBCD"
Use that to add GRUB to the Windows bootloader