Not easy to give "idiot-proof commands" for this kind of things!
First, try to disconnect any unnecessary devices (printers, modem, network cables, external usb/e-sata/firewire devices) and keep the strict minimum required connected: the keyboard, the mouse and the screen. It seems that such devices (especially external USB drives) can prevent the system from booting normally. I saw this in many mswindows computers, from win2000 to win-2008 server systems.
It is probable that something goes wrong with the vista boot code. It is usually possible to "repair" the damaged boot code from the windows install media (it was the case at least with previous versions, I don't know exactly with vista).
Another possible issue is some hardware problem in the hard drive, the motherboard, the memory, the power supply unit, ..., ..., ...
If a card (especially the graphics card) is not properly set in the slot, the computer mostly won't boot.
I think you could try to check if the computer works with some other operating system. To so this, you can use any bootable Linux Live cd/dvd, like Knoppix, Ubuntu, Fedora or others. They work from the RAM only. They normally boot into a desktop environment where you can even mount the hard drive, check if the hard drive is accessible or not. Furthermore, you can even copy files to other units, like external hard drives or flash memories.
GParted is a Linux live cd too, however the desktop screen from Knoppix or Ubuntu will probably be more familiar to you.
It is possible to clone the entire hard drive to another hard drive (same or bigger size), using clonezilla, a free Linux software optimised for this kind of copy operations (gparted can perform partition copy too, clonezilla is usually faster).
In the case of laptop computers, there are often one or two hidden partitions with a copy of the operating system and/or check software for the specific hardware.
If you just want to keep data files (text documents, photos, audio or video files), it is possible to just copy them to a new location, without cloning the entire hard drive. To save emails, you have to know where your email program keeps its files, and copy the entire folder.
If a live linux distribution can't boot, then it is probable that there is some hardware issue that needs a specialized service lab.
If the hard drive is good (even with a broken boot loader) but some other hardware problem keeps the system from booting, you could connect this hard drive to another computer (directly to the controller or through an external enclosure) to check it. Note that a USB connection is much slower than a SATA or IDE one, so that the various operations can take quite long.
(Moved to the live media section)
*** It is highly recommended to backup any important files before doing resize/move operations. ***
Thanks for helping out.