You'll need a larger capacity hard drive for Win7. This article recommends at least 40Gb click here.
I've successfully installed Win7 & running it on a 20Gb drive (desktop), but 40Gb is ideally needed. Performance starts to suffer when a drive is filled to 50% of its capacity & above.
Also, please note that some companies' warranties are voided if a different OS, to the one that it originally came with, is installed. G