It's not too difficult. Your first hurdle is to grab a copy of Windows 7. Do you know anyone else with a disk? If not there are sites like mydigitallife that regularly update links to download. See the site for details about how this is possible, and to answer the question of the legality of this. As laws vary by country, you should ask a lawyer if you aren't sure about what to do.
You need a Windows product key to use with the laptop. Sometimes the key is on a sticker on the bottom of the laptop. If it's not you might need to buy a key from Microsoft.
After you have a copy of Windows 7, and a key, back up whatever you have in the Ubuntu install. Install Windows first (This will wipe everything on the drive, including Ubuntu), then reinstall Ubuntu. Make sure you do it in that order. Windows first, then Ubuntu.
If you sort this out without too much difficulty, post an update with how it went. More people are likely to try Ubuntu if they can see how to switch back to Windows later.