Windows 7: Unable to install windows 7

After select Custom - format and delete patition ,then click next ,it show"Setup was unable to use existing system partition because it does not contain required free space.
But the partition selected contain 185 gb

please advise.

Check closely and I believe you'll find it created a 100mb System Reserved boot partition since you used the installer to create partition(s). Amongst other things, it will put your Repair Console available on the Advanced Boot Tools menu available by tapping F8 at bootup.

You can format that small partition once created, but then you need to use the down arrow or mouse to highlight the actual install partition underneath it, format it, then click Next to start install.

Try that to see if it helps.