Before I started installing Windows 7 Home Basic on my laptop I had 40GB of space on my hard drive, but after the installation I have no more free space available.
It's quite possible that system restore must have taken up a bit of space while you got Windows 7 installed. Try the steps in the below link to delete restore points and free up disk space. http://windows.microsoft.com/en-IN/windows7/Delete-a-restore-point
Did you install service pack 1 for Windows 7? In case you did, try the suggestion in the below link. http://www.intowindows.com/how-to-recover-lost-disk-space-after-installing-sp1-for-windows-7/
Finally, if you installed Windows 7 on top of an existing installation of a Windows OS (say Windows Vista), the installation may have prompted if you wanted to keep your older files. In case this has been done, look for the files from your previous installation and decide if you want them removed.