$100 laptops are available but they are only useful for typing and saving basic documents. If you want to get online and surf the internet then you're going to have to bump up your budget to at least $299. Ebay is the only place I can think of where you could find 100 dollar laptop computers. Keep in mind that $100 laptops are old, used, discontinued machines that usually contain outdated components.
There are brand new desktop computers available that start at $199 and are suitable for browsing the internet, checking and sending email, listening to and burning CDs, watching DVDs, and downloading or saving a reasonable amount of files. The cheapest laptops typically go for just under $400 although Asus does produce a 7 inch model that starts at $299(only has 2 GB hard drive space). If you want the cheapest computer possible and want to be able to surf the internet safely then my recommendation is to consider the options outlined further below and go for either a $399 full-size laptop, a $299 mini Asus