How to erase the hard drive on my laptop but keep windows?

Uninstall all programs you do not want to leave on the machine.

Go to control panel - Users and create a new user account in a different name. Make sure it is an administrator account.

Restart the computer and select the new account to log in to.

Go back to users and delete the old account, choosing not to save files.

That will remove you old account and all the associated files and data.
To be sure, restart again and from within Computer or Windows Explorer, go to Drive C, Users and check if a directory still exists for your old user name. If it does, delete it.

Then get ccleaner and run that to remove all the temporary files and junk. It also has a facility to 'wipe free space', which over-writes all deleted files so ensuring deleted data can never be recovered.

It's free and has no adware or nasties.
http://www.piriform.com/...r/download

Another option is to remove all unwante dprograms, data, etc. Follow this with a full drive defrag, then use something like FreeWipe to wipe ONLY the free space of the drive.

Use Hiren Boot CD, it contains many partition softwares in it. Boot with your USB and delete other partitions and leave the windows partition. You can also recover files using Mini Windows 7 Mode in Hiren Boot CD.
It will load Windows 7 OS with least features.