Fuzzy monitor, computer won't boot up?

I have a Compaq Presario desktop, model SR1710NX. It came with 256Mb of RAM, which I just recently upgraded (after a couple of failed attempts) by adding another 1G stick of DDR RAM. It worked for a few days, except that it wouldn't come out of hibernation (I would have to force shut it down and then start it up again). One day when I turned it on, the monitor was fuzzy, and wouldn't boot all the way ("boot disk error"). I turned it off, and then back on again, and it worked; however a day or two later it just stayed stuck that way.

I contacted HP tech service, and with their help found that the BIOS was not detecting the hard drive. After reseating the HDs power connections without change, the tech said the HD must be corrupted.

I question that assessment, though, because that does not explain the fuzzy screen (which showed up at exactly the same time).

I understand that I might just have to get an expert in to do some actual tests on the machine, but any help would be greatly appreciated!

BTW, I did also try to retest it using only the original RAM.

Thanks!