I got error messages indicating that my HDD was dying,ran a couple of "Read/Write" tests and they showed my poorly HDD as clear.The only software that seemed to be having trouble with it was XP,but then when I tried to reinstall an image taken of this hdd,it also kept giving up errors.I gave up the struggles and replaced my IDE hdd,ran various tests on the "dead" hdd,and every once in a while,the test failed,or produced "dog slow" readings.I am not going to "trust" my poorly hdd to carry the OS,simply use it to store data(that can be replaced with ease)until it makes up its mind and either quits completely or ceases messing me about for a good length of time!
Look in your BIOS - and look for the item "Quick Power On Test". If it's Enabled, then Disable it - This will run a longer RAM test when you power up, and sometimes shows up faulty RAM right there and then.
I had a system just like this recently - pass the quick test, but the system kept screwing up - enabled the longer test and it threw up a RAM failure right away.
It's not perfect, but it's better than no test at all.