Memory issues

Did you install the RAM in slots 1-4 with slot 1 being the farthest from the CPU? You could also try replacing the CMOS battery as with a system that old the battery could be giving you problems. Use a brand new recently purchased battery. One more thought is that 16GB of RAM is not listed on the spec sheet as a compatible RAM total. It may be fine but just be aware of that fact.

I probably should've seen that a little sooner - In spite of max spec RAM, you're right, 16GB isn't listed as a compatible configuration whether it's 1 or 2 CPU's.

Sorry @bradthetechnut  if my reply was confusing. The system can take well more than 16GB and I knew that from the specs. My point was that 16GB specifically was not listed as compatible on this page. I see 12, 24, 32, and so on but no mention of 16GB. Again sorry if my post was vague, imagine what my family and wife go through on a daily basis.

I think the memory issue with OP' setup are:

• Memory were populated incorrectly on DIMM slots.  Should use 3 sticks instead of 4.  The supported configurations are mostly triple channel.
• Using the wrong type of memory.  Should use RDIMM, instead of UDIMM

Thanks for all the replies and thanks @bradthetechnut for the guides.  I can see now why I'm having issues, I was using non ECC memory and that was before incorrectly populating the sticks.  I've managed to get it to boot using the existing 6x2gb DIMMs but it crashes soon after.  When running the BIOS memory diagnosis on the this memory all seems good but during POST it will sometimes flash up very quickly Alert! Uncorrectable and that's all I can manage to read before it disappears.  This message I think indicates in what stick the problem occurs so does anyone know how i can pause this message when it displays (which is hardly ever).  Although as I've been spending most of the last few days swapping out various combinations of the existing memory and it still crashes they are probably all suspect and I should buy some new, correct sticks.  Thanks for your help everyone.

I think new correct sticks is the way to go too. 

Still might want to replace the  battery.

If you had another PC, not that it would work for the T5500, but you could test the non-ECC RAM one stick at a time.

About the only way to find which ECC stick is bad is too test it one at time.  Unfortunately, 1 stick isn't a supported configuration in the T5500 (as you likely know by now).  Would need another PC that supports 1 stick of ECC RAM.