Ray,
A lot of things could be causing your problem, but mostly I would
concentrate on the hard drive sub-system. Having said that, you should
immediately back up anything on the drive you don't want to lose.
Here is a list of some of the more likely culprits in a "least-cost" order.
1. Loose IDE cable....re-seat the connectors.
2. Bad cable....replace it with a known good one. Make sure it's an 80 wire
(Ultra ATA compatible) because I suspect a machine that new uses the faster
interface.
3. Bad IDE interface....It's part of the motherboard but you can get PCI
cards that do the same thing and still use existing motherboard.
4. Hard drive about to die....you might want to try booting from a floppy
the next time it won't boot from the hard drive....if it boots consistently
from the floppy it eliminates just about all of the other sub-systems from
the problem list.
The memory could be flakey, but I wouldn't mess with it until you eliminate
all the others. It seems like everything is going ok until the BIOS tries
to hand off control to the O/S, which is entirely on the hard drive.
You might try to access the hard drive after booting from the floppy. It's
possible just the MBR (master boot record) area is getting flakey. Without
actually fiddling with the system though, best guess is the drive is
getting ready to give up the ghost.
Michael

>  Once a week lately on bootup I get two BIOS beeps and message "No operating
>system found." I take the tower apart, pop out the memory, reinstall memory
>and computer stars with no problems. Is this a bad chip, dust "I cleaned the
>case regularly" or is the motherboard going bad?

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