Well, this seems like my kinda problem ^u^
As you stated above, it might be due to old age, but I would be betting on the dust. Due to it gathering around the computer it is know that sometimes it can mess with the hard drive. The hard drive is pretty much a solid peace of metal with a magnetic reader. If dust gathers around it the dust gets polarized and sometimes can cause it to corrupt the data on the hard-drive. Hard drives are built to be sturdy especially old SATA types, since I also have a PC that I bought back in 2001 and its still running with the factory Hard drive. If you can still access it by any way, such as Safe Mode, try to backup the data, and then do a clean format on the hard drive.
If you have your Windows copy, you could try running it and see if you could repair the corrupt or missing files. If that doesn't work, then you could try a clean format, but you would lose all data that is stored on the hard drive.
If you solve the problem, I would recommend trying to get Windows 7, due to the fact that it has a recovery mode and is way more protected than the previous version of it.
And a little tip to avoid future problems like this. Try saving your most important data on external hard drives or flash disks. Even a brand new Pc can sometimes crash without giving a single warning.