For the past number of months I've experienced intermittent crashes of my computer. The computer would shut down completely and restart. The computer programs hang for about 1 second then the power switches off and the compter reboots. There is no warning or shut down of processes. This can happen a few times a day or not at all for over a week. This MAY appear when I am actively using the computer or after the computer has been idle for a few hours.

When I allow Windows to send a report to itself, the message on their page states that the PROBLEM is with random access memory.

I've tried the Windows memory analysis tool but nothing is ever found.

My system is set up like this:
Operating System: XP Media Center Edition SP2
RAM installed: 2 GB (I added a new 1 GB DIMM a few months before the problem started)
Proscessors: Dual Pentium 4 - 3.4 GHz
Security: McAfee Security Center (The problem existed before this install)

I've kept the computer up to date with Windows Update and have used various security programs for spyware and viruses. (Microsoft, TrendMicro, LAVASOFT and McAfee)

The computer was purchased in October 2006.Sounds like the new RAM is not compatible with either the other stick or the machine itself.
Travel to Crucial.com and run a scan of your machine and it will tell you what type of RAM it takes.....all RAM is not the same.Both DIMMs are identical. Synchronous, 1024 MB, 533 MHz, 64 bit total and DATA width. Different manufacturers though.

The new DIMM worked well for more than 2 months before this problem arose.

I'm thinking that the problem is actually caused by some software conflict.
I have seen instances when different manuf. sticks would not play together.
You may want to DLoad and run MemTest and let it run a few hours.I ran the Microsoft Memory test CD and ran it for a number of hours with no problems.

The crashing had escalated to every 10 or 15 minutes.

I REMOVED the extra DIMM and the problem has not recurred.

Thanks for the help.Try Memtest86 instead, it is more comprehensive than the MS memory test disc.