I see this general question posted here a lot. Most people who answer insist that windows will happily delete the content of your hard drive without further intervention by linux, and in the past I've known that to be the case, but considering the rate at which people like you seem to be asking, it only makes sense that I at least cater to you with the easiest and fastest solution I've proposed to anyone. As I'm not certain the question I answered earlier today is going to get deleted, I'm just going to repost my answer here.
The super simple way to wipe your hard drive to like new status (no partitions at all), is to boot your computer to literally any ubuntu Live Boot boot (media) CD/usb-flash. It doesn't have to be remotely up to date, so whatever you installed ubuntu with should be more than adequate.
Once your computer is booted to a live boot CD you merely open one program already on the live boot media. It is called Disk Utility, and it will do exactly what you want to do with absolutely no thinking or grief. You don't even have to open terminal! Detailed step-by-step instructions below for both opening the utility and what to do once you have it open, but my guess is that you could adequately wipe your hard drive with that utility even without the following instructions.
Dumbed down usage instructions:
1. Open Disk Utility (press the windows key on your keyboard and immediately type DISK and then press enter)
2. First select your hard drive from the left side menu within that window.
3. Then maximize the window so you can see all its options.
4. Choose the lower right option called FORMAT VOLUME.
5. In the window which appears, change the selection in the drop-down menu next to "TYPE" to EMPTY. (please don't try to give it a name. You cannot since there is no partition there to name)
6. Press format, and the drive will be completely wiped, devoid of data, partitions or even partition tables.
This type of erasure is not quite the same as SHRED nor is it as time consuming. If you turned your drive over to the CIA they could probably recover all your data before sending you to a secret prison... or perhaps afterwards, I won't speculate. If you want that type of file destruction and have the time, I recommend shred instead. The method I've described is much faster and will ensure that windows will install correctly.
One last item I'd like to mention. When you first open Disk Utility, it will offer you a peek into the "SMART" status of your drive, and can tell you if your hard drive is healthy. All operating systems which install to hard drives rely upon the hard drive working correctly. If you have had an unhappy experience with Ubuntu, there's a good chance the cause was actually that the drive is sick. In which case, Windows won't work right either.