1 Sorry to hear about the problems with your Vista computer, As you know Vista is well into its final year of receiving support from Microsoft, so it might be wise to consider if its worth repairing the operating system just for a few months usage, or whether you might be better off installing a more up to date OS either Win 7 if you want to continue with Windows or perhaps Linux as of course Linux is free...
2 It looks like you have tried a number of things so far, but the easier repair method by far is just to reimage your hard drive with a recent disc image made when the machine was working fast and sweet.
3 Reimaging will have you up and running again in less time than it takes to walk the dog!
4 Assuming that your data back ups are all 100% up to date and reproducible you could reinstall Vista either using a Recovery partition if one was included with your computer or a fresh install using the correct Microsoft Vista Installation disc together with the 25 character alpha numeric product serial key that is usually attached to the computers case
5 If there is anything on your computer that for one reason or another may have escaped being included in your latest back ups then I would be inclined to fix that before going any further.
6 if you still can't boot into Vista, safe mode, or get access to a command prompt (with elevated privileges) then boot into a live distro of Linux (Linux Puppy is good) and fits nicely on an ordinary CD, adjusting the BIOS if necessary to boot first from your optical drive and second from hard drive0, boot into Puppy using the CD and use it to pull off any important stuff prior to the fresh install of Vista.... (you can copy your stuff to an external hard drive or memory stick whatever is the nearest to hand.
Puppy will run quite happily in RAM without writing to your hard drive (unless you ask it to!) and of course when you reboot the Puppy will be gone.. until you need him again!
7 Do remember when burning the Puppy bootable disc you will need to download the .iso and burn it as an image.... an ordinary copy just won't do!
Post back if you need any amplification on any of this