I've installed Lubuntu on my computer, but I let to be Windows primary. How do I get to Lubuntu?
I have no offer list while booting Windows, Windows is booting automatically without chance to change it to Lubuntu.
Judging by your description, it seems like your harddisk's MBR (the first sector, used to boot up from that harddisk) points to the Windows bootloader. The Windows bootloader is incompatible with most operating systems other than Windows itself though. This commonly happens when you install Windows after Linux, or if you "repair" Windows.
What you want is a bootloader that is compatible with both Linux & Windows. For instance, GRUB, the bootloader used by default by Ubuntu.
Updating/Reinstalling GRUB, should therefore fix your issue.
This answer explains particularly well how to proceed: http://askubuntu.com/a/88432/29365
You will need a bootable USB or install CD.