fitted new hard drive to compaq c700 how come my disk wont boot ?

Cause:
Apparently, you installed it as the primary drive. A new drive don't have ANYTHING on it. The primary drive absolutely need to have the ntldr file as it's that file that enable you to use an NTFS formated drive.

Solutions:
If you still have your original drive installed, as some laptops can:
You can try swapping the connections between the two drives.
It should place the original drive back as the primary drive, the one that is used to initiate the boot sequence.
In this case, you don't need to reinstall anything.

If it's the only drive installed:
You need to install Windows from scratch. It will NOT be a reinstall but a CLEAN install.
OR
You may want to install Linux. There are many distribution to chose from and almost all of them are free and can be downloaded from many sources. Ubuntu is easy to use, but you may prefer another distribution.

Electro
July 2012