Crashes caused by exception #'s 80000007 and c0000005?

I recently built a pc for the first time and while it runs pretty well for the most part I have been having problems with random program crashes (including firefox, explorer.exe, games, etc) every once in a while. I initially thought it was probably a problem with the ram, and ran memtest for about 4 hours but it found no errors. I know that errors can still exist even if memtest doesn't find them, so I tried taking out a one gb stick (I have 2 gb installed) to see if the problems would persist. They did, so I switched to the other stick and still had problems so I ran memtest again with the one stick installed and got no errors again.

I just checked my Dr Watson log (which is now pretty huge) and found that all of the crashes have involved exception numbers 80000007 and c0000005 (almost all being c0000005). I know that that c0000005. If anyone has any idea what could be causing this error I would appreciate it.

Here is my setup:

Operating System: Windows XP Home (SP3)
Motherboard: Gigabyte GA-MA78GPM-DS2H
Onboard Video: ATI Radeon HD 3200
CPU: AMD Athlon X2 5000+ Black Edition overclocked to 3.0 GHz
Ram: 2 GB OCZ PC2-6400
PSU: Antec Earthwatts 500 W
HDDs: Hitachi Deskstar SATA 320 GB (OS), WD 300 GB IDE (Storage)