|"I discovered the fan was mounted backward (at the factory); turned it 90 degrees but to no effect." That actually would be 180 degrees if you turned it completely around from blowing in to blowing out of your case (the fan should be set as exhaust to get rid of the heat).|
Run Memtest86 from bootable CD to test your memory.
Test if you purchaed the correct memory by running test from crucial.com.
Run Seatools from Seagate via a bootable CD to test your hard drive if you still think it is an issue, but you will probably need a larger hard drive when you switch over to Windows 7.
Run Malwarebytes to test for an infection that may have slipped past your antivirus program.
Uninstall any program you do not use.
Do a disk clean up.
Do a disk defragment.
Blow dust out of your system using a can of compressed air and make sure that your front and rear vents as well as your heat sinks are clear of dust. Vacuum only outside, after you reclose the case.
Run msconfig, start tab and uncheck all except Microsoft and your antivirus program, Apply and restart the machine.
I agree that adding a video card would be an advantage if you switch to W7
On power supplies, look for ones that have all of their 12Volt amperage on a single rail and is certified 80% efficient. These are not your $20. ones, they are usually $40 or $50 or more. The cheaper power supplies fake their wattage by pushing the amperage of the 5 Volt rail higher, but you can never use this since all of the high draw components rely on the 12Volt rail for their main power requirements. You can in practice get a 350 Watt power supply that is not really an upgrade from a 275 Watt model. If you do not plan on gaming, then an inexpensive modern graphics card will do as will a decent 350 Watt power supply, but if you do plan on gaming, plan on a gaming grade graphics card and a power supply that is appropriate for it.
You have to be a little bit crazy to keep you from going insane.