DELL Dimension E510 won't start. Lights 3 and 4 on

Has anybody out there found a solution to this problem?  I've had my E510 for six years now.  Replaced the PSU a few years back when it went out and this identical problem just came up.  

I put in 4 new-to-me sticks of RAM, basically 2 matched pairs of 2x1GB for a total of 4GB and they have been working just fine for two months now.... until I started getting the 1 and 4 diagnostic lights with the boot screen coming up at about 10% on the bar.  Just today, the 3 and 4 lights came on with the drive/disk icon lit up as well.

Everything I've read in the forums points to the motherboard.  Is there a way to test it without pulling the mobo?

Any help would be appreciated.

The only sure way to "test" the motherboard is to replace it with a known good one.  Where is it failing?  At POST (Power On Self Test) or when it's trying to load Windows?  If it's at the POST then there is no way to test anything.  If it's failing when loading Windows then you can run the Dell diagnostics (F12 at POST to access the diagnostics) and see if anything turns up.

Thanks for the response fireberd.

It has been failing at POST, I was able however on one attempt last night to boot up Windows XP,  I ran the Dell Diagnostic test at F12 but it seems to only allow the option of diagnosing the hard drives not the entire system.

I was able to look through Device manager in the Control Panel and nothing showed up with yellow flags there.

System ran for about half an hour before blue screen crash with NAVEX15.sys showed up.

I was able to run a diagnostic scan with Norton Antivirus 360 and nothing showed up as faulty.

Also ran a scan of memory sticks and processor with a program called Hot CPU Tester  hot-cpu-tester.en.softonic.com which showed everything working normally.

This morning, tried booting up and I can only get to the Dell boot up screen showing the BIOS status bar stuck at 10% once again with the 3 and 4 diagnostic lights on.

I don't have access to a "good" motherboard right now, I'm looking however at option of buying a matching cheap used motherboard off Craigslist or ebay to get me by for now, or just outright purchasing a new computer instead of getting nickel and dimed to death with a five year old computer that is bound to die soon anyway.

The E510 is a BTX style - most motherboards are ATX and will not fit/work.  Also, you need another 5150/E510 motherboard due to the Dell proprietary (and undocumented) front panel connection (which can vary from model to model).  If the one on Craigslist is for this model then it will fit (whether it's really good or not is another issue coming from Craigs list).

A little update here... I removed all four sticks of RAM and then powered up the system....it gave me that beeping noise so I assumed the mobo was not the only culprit here.  I then proceeded to boot up with one stick at a time.  

Was able through the process of elimination to detect one faulty stick of RAM out of the four.  

When that one stick was added the BIOS screen would show up and the meter would not progress beyond about 90% when it was added with other good sticks of RAM.

For now at least, the system is up and running.  I'm still unsure of why I was getting 1 and 4 lit up before and then 3 and 4 after a few days of unsuccessful efforts.

Plan for now is to save up and replace this aging beast as soon as possible.

Thanks again for the helpful feedback.

I work on pcs and macs for fun and I ran into the same problem you have on one of my church members pc. After hours of looking for solutions and nothing solving the problem, I removed the 1st and 2nd ram slots and it booted right up. I believe an upgrade of ram was made with an increase higher than the motherboard could handle. It had been working with 2048 but ceased to function until it was using 1024 again. I hope this helps