Replacing hard drive in Aspire One series is a bit complicated, so you may want professionals to handle it for you. If you want to do it yourself, this service manual will be handy. Disassembly instructions start on page 44.
Make sure battery is removed. HDD case is located under the keyboard. Removing LCD panel isn't necessary. Replace HDD and reassemble your netbook carefully.
As @Ramhound said in his comment, it's very likely that you already have a Windows 7 license - look for a sticker on the bottom of the computer. If you don't have one, you'll have to purchase Windows 7.
Finally, you'll need another PC, a 4 GB (or bigger) flash drive and Windows 7 DVD or ISO image. Make sure to download the right image, as the serial number from your license sticker will work only for specific edition of Windows. If you have a DVD that you have purchased with license, it will suffice.
Use the Windows USB/DVD Download Tool to create a bootable flash drive. It will ask you to insert the DVD or select ISO image. When the process completes, connect this flash drive to netbook's USB port and boot from it. You'll probably have to press F12 on the initial Acer logo screen, then select flash drive.
The process of installing Windows 7 after booting from flash drive is identical as if you'd boot from DVD. It's pretty simple and well documented, just use Google if you encounter any problems.