Do you own a copy of windows on a disk and an activation code for windows? If so, make sure all the files you want are off your computer somewhere else. Put in the cd. Turn off your computer and power it back on. A screen should pop up before ubuntu telling you to press a button to enter bios, press it. If it does not show up, try again and hold down delete as the computer boots. The specific bios varies by manufacturer, but you want to boot from the disk drive to run the disk. You may need to look up your specific motherboard. From there, windows will walk you through set up. There are a lot of tutorials for installing windows online if you need help.
If you do not have the activation code, you can only have windows for a month. Dual booting is not so simple, what I described will wipe your harddrive clean; although a harddrive recovery system could still get files off it, so this will not hide incriminating evidence. You some sort of gamer, or do you just hate open office? Well, I personally like windows, but the hundred dollar price is too much for me.