So while installing windows on my system a few months ago I left ~40 GB to install ubuntu later and now I wanna do it,so is there any way I can use the Windows Installer to install ubuntu on that unpartitioned free space or I need to install it in the old fashioned way on my current windows installation
WUBI won't work for you in this situation. WUBI makes a disk image file on your Windows partition to store the Ubuntu OS and then mounts the disk image when you choose to boot Ubuntu. This gives the benefit of trying Ubuntu without needing to Repartition the HDD and only suffer some very minor limitations. Plus it let's you remove Ubuntu like it was a regular old program.
Since you already Partitioned the Space ahead of time you might as well use it. Burn an Ubuntu Installer DVD or make a Bootable USB and then just install Ubuntu using that.