Boot trouble

I have an older PC with an ASUS A7M266 M/B running Windows XP Pro. It started shutting itself down and rebooting. It would run for about ten to twenty minutes and then just "blink" off and reboot. I assumed I had a bad stick of RAM or that something was overheating. I changed out the RAM and cleand the fans and heatsink but it made no difference. I didn't power it up for a few days and now it won't boot at all. After POSTing I get the "error loading operating system message". It won't boot from the CD drive or the floppy either. It just hangs when I try to boot off the CD with any known good disk (Windows XP, Linux and UBCD). When I try to boot from the floppy with a DOS boot disk I get the "invalid system disk, remove the disk and press any key to continue" message. The same floppy boots OK on two other PC's. I can get into the BIOS set up and everything seems to be set OK. I haven't made any changes or added any hardware in years. I did notice some weird spelling errors on some of the screens though. I tried clearing the CMOS but that didn't help. All the drives are identified when it POST's. Is my BIOS toasted? Is ther anything I can do?

Tried that. It never gets to the first console. It just POST's, prints "booting from CD" and sits there - no activity. The same sort of thing happens with the floppy. It sounds like it's reading the disk and then says "invlaid system disk". It's like it can access the drives but it doesn't know what to do after that.

Just tried changing both the power supply and the RAM. It made no difference. So far I've changed the RAM, power supply and the video card.

Try Ebay, you should easily be able to find a replacement socket A mobo really cheap. However, if you have the cash, you may want to look at the barebones bundles you can get as anything on socket A is pretty long in the tooth nowadays!