Does the system tell you how to enter the bios? If you're seeing the message "operating system not found", you've gone way past the point of entering the bios (CMOS). You have to do that when the computer first boots up. Usually you hit ESC, DEL, or F3 immediatly after the POST to enter the BIOS. (If I don't get clear instructions on which key to push, I start pushing each one in rapid succession)

I had the very same error message with my PC, mostly after windows would boot. then It occasionally gave me the no OS message, and sometimes no BIOS.

But before you read on and think, right, "I'll copy this guy", don't! Seriously! Go through a checklist first, look stuff up according to what you see, research it, analyze, and use a process of elimination! Don't just destroy and rebuild!

it's either a Hardware problem from one of your components, outdated drivers or System/Hardware BIOS/Firmware, or you hard disk drive is on it's way out! If it's louder than usual, then it's probably scratching itself to death or something like that! it could be

I'm not sure what solved it, but I took down and searched forums vie Google for the various BSOD errors that I saw online. I read that the best thing to do was using trial and error by removing all hardware (PCI Sound Cards, Extra Drives, Network cards, USB docks, etc) except the essentials, for example my Graphics card, which was needed to stay put as it's not built in on my PC.

It's all good and well to try this first, but I recommend putting down on paper everything you do and see to back track just in case, and to help you pinpoint the problem, even though I just used my memory... no not RAM, my brain! And of course backing your game saves, your "c:\documents and settings" folder, program settings and stuff like that.

I just kept rebooting with different hardware installed (or removed) every time I booted again after any familiar errors occurred. So I'd try various configurations with the hardware.

For me, It seemed to be a hardware problem, that's what the forums said, so I used a different Hard Drive, and, because my Graphics card has no fan built in, I checked the heatsink and nearly evaporated my finger! So I used a system exhaust fan underneath it.

System Exhausts are cheap to buy from most decent PC hardware shops. They fit in the card bay and connect to the Hard Drive/CD Drive power connectors. Don't worry about losing a socket though, it is a throughput socket so you could still use two optical Drives and Hard Disk Drives together!

My PC uses an MSI Motherboard, and thankfully these guys have a nice idea which makes it easy to upgrade it's internal chip stuff, AKA Firmware/The technical term is BIOS.
But anyway, it's called Live Update 3, and it works a treat, well at least it did, when it worked after re-installing windows on the swapped over Hard Disk Drive (the other disk really didn't want to be formatted most of the time, and when it did, installations crashed, so, yeah, that could well have been the problem in hand.

So, OK, the updates worked, then I installed Anti "spy ware" and "Anti virus" stuff (I recomment AVG and Spybot), oh and a firewall, because as you might know, using Internet Explorer after installing XP makes your PC very very vulnerable, like a newborn baby!
Next, was the Graphics Drivers, most important, these count for many system errors and crashes these days, my dad told me, and it's so so true! Then after "Windows Update"-ing windows (yawn), I was away!

I think my biggest mistake was not doing a regular Hard Drive "Defragment", and not downloading Download.com's update checking utilities and/or Driver/Software update Email notifications, it may have saved me more time before I got into this mess!

I'm not saying anyone with this problem didn't, but I do suggest you take in my advice in the second paragraph up there!

Good luck and may we all one day have a Quad Core, Dual Socket, Toy Story type Graphics PC this Christmas!

>.< God, give me more money, now!