Here are some things to consider. Apologies in advance if you've already worked through them all:
- You could be looking at some sort of spyware/malware/virus mucking with the files on disk. There are a number of free or inexpensive anti-spyware and anti-virus tools out there; it might be worth running a scan.
- This could be a disk going bad. You could try running tools to scan the disk, though, since you say you've had problems with hard disks in the past, that seems less likely.
- It could be a bad I/O controller or chipset. Things like this can be very hard to isolate and track down. Things to look at include the temperature at/around the northbridge. You can also try running benchmarking and stress tools like Prime95 (http://www.mersenne.org/
); when I was having stability problems on my last machine, Prime95 would die pretty consistently very early on. It was a good parakeet.
You can also try games like putting a hard drive on a different computer to see whether it acts up there, but of course, if it's your boot/OS drive, that's obviously easier said than done!
- It could be, as others have mentioned, a bad power supply. This, too, can be tough to isolate. I'm not sure who makes the motherboard in your Presario, but if they make some sort of motherboard monitoring software, you might try that. Personally, I absolutely love Intel's motherboard monitoring utilities; they show me power levels on each rail (5V, 12V, etc.), temperature at several zones around the motherboard, etc.
- I would generally consider bad memory, but others have already covered that ground.