In Linux you can come to know each partition has a type number. NTFS is type 7. Linux is Type 83 and Type 82 is swap. You can delete the last two if Ubuntu is the one you want to delete. A good OS will not allow you to use it to destroy itself so you need to boot up Ubuntu Live CD and use it to remove the installed version in the internal hard disk.
When you run a Linux Live CD the Linux is in the ram only and in the CD so it doesn't mind to destroy whatever you have in the internal hdd. Got it?
In Win7 if you right click "My Computer" anywhere, then double click Manage, then storage, then disk management you should be able to see all the partitions in the hard disk. Windows does not support Linux so Ubuntu partition does not have a drive letter. Highlight this partition, right click and select delete and it will be gone in 2 seconds flat.
If you cannot recognise the partition in Win7 then boot to Ubuntu, click Application, the Accessories, then Terminal and show me the output of