I'd imagine that just about all the large vendors allow you to download and purchase online nowadays.
There are plenty of free options too, such as Microsoft Security Essentials, Avast, AVG. On a netbook I'd be tempted to go with the lightest which is probably the Microsoft one.http://windows.microsoft.com/en-US/windows/products/security-essentials
Used all three of the above in the past, currently been sticking with Avast as I quite like the interface and the modular sort of approach.
In short, nothing special required for a netbook apart from choosing something a little less resource hungry.