Download the ubuntu iso , prepare a live usb,its fairly easy. find how to do it,yourselves.
Go to windows 7 , press winkey + R type diskmgmt.msc,press enter.
1)delte the D drive , so that it shows up as unallocated space.
make sure you apply the changes.
2)Now restart the computer go into bios, usually it is done by pressing f2 or f10.. find the boot sequence and set the usb on top.(in short boot from the damn usb)
NOTE:An yeah don't forget to change it again to the hard drive when installation is complete.
3) A "window"or screen will open,press enter on the optiopn that says try without installing.
After booting in ubuntu , press the window key and type gparted, open it up.
Dont worry about the confusion of sda1 and all.
If you aren't sure then refrain from doing anything,without seeking help.
Now Press on the unallocated space,and change...set the ENTIRE SPACE AS EXTENDED PARTITION.
after doing it you will still have unallocated space from that space do the following
all of them are LOGICAL PARTITION.
1)set the size as around 30Gb(approx 30,000), choose type as ext4 , mount point as / .
this is where ubuntu will be installed.
2)again press on the remainig unallocated space, set the size around 2gb(approx 2000) , choose type as swap.
3)press on the unallocated space, change size to the maximum, this is where you will be storing your FILES/DATA. choose type as ext4 and mount point as /home.
Apply the changes and close gparted.
If you aren't sure what you haev done then , post a screenshot of gparted after doing all this.
If you have come this far , then go to the desktop press on install ubuntu icon ,when the option comes choose SOMETHING ELSE and choose the drive which has the partions you just made above and bingo! proceed to install.
Good Luck.