Read your minidumps and here is what they cited ...
One error is 0x0000007E: SYSTEM_THREAD_EXCEPTION_NOT_HANDLED
A system thread generated an exception which the error handler did not catch. There are numerous individual causes for this problem, including hardware incompatibility, a faulty device driver or system service, or some software issues.
The driver cited is nv4_disp.dll and is a Nvidia display driver.
Another error is 0x00000050: PAGE_FAULT_IN_NONPAGED_AREA
Requested data was not in memory. An invalid system memory address was referenced. Defective memory (including main memory, L2 RAM cache, video RAM) or incompatible software (including remote control and antivirus software) might cause this Stop message, as may other hardware problems (e.g., incorrect SCSI termination or a flawed PCI card).
The driver cited in this one is again nv4_disp.sys
The remaining two are 0x0000007F: UNEXPECTED_KERNEL_MODE_TRAP
One of three types of problems occurred in kernel-mode: (1) Hardware failures. (2) Software problems. (3) A bound trap (i.e., a condition that the kernel is not allowed to have or intercept). Hardware failures are the most common cause and, of these, memory hardware failures are the most common.
The one driver cited is the Nvidia miniport driver nv4_mini.sys
The other dump cited memory corruption. You ran Memtest well. Did any errors show up at all? As good as memtest is bad memory has been known to pass it on occasion.
So, here are two suggestions:
1. XP because it is a 32-bit operating system will not recognize 4 sticks of RAM and neither will 32-bit Vista. The most is 3 gigs. Only Vista 64 bit OS will utilize 4 gigs or more. Try running with two gigs and see if that brings stability.
2. Uninstall and then reinstall your Nvidia drivers.
a. Download and install the free version of Driver Cleaner to your desktop.
b. Download the Nvidia drivers you want to install but don't install at this point.
c. Uninstall Nvidia drivers and then reboot into Safe Mode.
d. While in Safe Mode run Driver Cleaner. If it doesn't find anything then that is okay.
e. Reboot in normal and install the new drivers.
*** Let us know what steps you took and the results.