Hi Jonzyy,
Welcome to Vista Forums! I'm going to give you the best advice I can given the circumstances you revealed in your post and knowing nothing else about the situation (which makes it very difficult because there's so much information not provided and/or not available under the circumstances).
If you can't boot normally, can't boot into any of the safe mode options (I assume you've tried them all - if not, do so), and can't boot to CD/DVD then you essentially have no way to access the system. If you can't boot from CD/DVD, then the issue isn't the hard drive because that isn't necessary for that type of boot - so slaving the drive to another computer for testing will not help but it may allow us to access some report logs which may help identify the problem. It could be a number of hardware problems (motherboard, CPU, processor, graphics card - though that's unlikely to prevent a DVD boot or a safe mode boot, or even the power supply) but those are very hard to test without replacements to swap out or special equipment unless the system detects and reports these in log files. Do you have another PC to which you can slave the hard drive to see if we can access it to retrieve some diagnostic and reporting logs? If so, and you can access the drive, post back and we'll advise you what to copy and attach to a subsequent post.
If you can't boot in any fashion, including CD/DVD or USB drives, then there's no way to update or flash the BIOS because you can't get the new BIOS files to load. If the computer manufacturer has updated BIOS available for your computer and your version of Vista, then by all means update them to see if that helps (maybe on one of the days your computer decides it's going to work if such a day occurs again).
If all this sounds like too much hassle or too complicated or you don't want to wait for your computer to decide to work again (if that ever occurs again), then I recommend you take it to a reputable computer repair shop (NOT Geek Squad or any of those other big store centers) where true professionals can service your machine (or you can send it back to the manufacturer for service). If you have data you want to save and don't already have a good and recent backup, then the slaving process described above is about the only way to do it (if it works) and you can then copy your data to the other computer or to an external HDD attached to that computer or to a DVD on that computer. I suggest this as the troubleshooting process frequently ends up wiping out everything on the hard drive as they try to find the cause and formatting and re-installing a working OS is a natural step in the process.
I hope this helps. To be perfectly honest, it appears from your description to be some sort of hardware failure (though it it still possible that it is software, but not as likely). Where we go from here is pretty much up to you and how you decide to proceed (or if you have the resources, skill, and/or patience to try some of the options provided). At this point, even I might consider a computer repair shop (or the computer manufacturer) to be the most reasonable answer even it it isn't the cheapest potential solution.
Good luck!
