Okay, I'm a bit knowledgeable with pooters and such, but i'm confused.  My computer is dying slowly, and I'm not sure what part is causing this. 
Computer details:  Vista, dell machine, intel Q6600, 2.4 Core Duo (quad core), standard memory and drive (unknown manufacturer).
Symptoms:  I would best describe the symptoms as memory corruption.  After a couple days on, I start getting applications crashing or failing to open for a lack of "resources".  Sounds are corrupted.  Onscreen text gets corrupted; the characters of text are garbled, not the pixels on the screen.  Video memory seems untouched as I haven't seen any misplaced pixels.
Recently I've lost files on disk.  I've also experienced errors reporting a supposed lack of disk space, even though I have fifty gigs free.
There was one point where I couldn't get to the POST when booting up.  After I cleaned everything (see next) this hasn't happened.  
Diagnostic steps:  First thing I did was clean the case.  There was a lot of dust buildup on heatsinks, so I cleaned all that up.  No help.  Next, I disconnected and reconnected everything, from power cables to memory (did not reseat cpu).  No change.  Last, I ran the standard vista memory diagnostics and ran checkdisk.  Both reported no errors found.  I have not run any POST tests, now that I think about it.
I'm at a loss at this point.  Disk appears fine, memory too.  I'd expect motherboard issues to result in the thing not booting up, yet it does every time.  
What should I be looking at?  What more can I do?