I have read through a hundred Dimension 2400 memory upgrade postings, but none seem to address the problem I am experiencing. If I install both 1 Gb modules and press the start button, the computer starts for about 1 second, then shuts down for about 2 seconds, and then starts up again on its own, but the BIOS screen never appears and Windows never starts.  If i replace the 1 Gb module in Slot 1 with the 512 Mb module and leave the other 1 Gb module in Slot 2, it starts and works fine with 1.5 Gb of memory installed. However, if I install the 1 Gb module in Slot 1 and the 512 Mb in Slot 2 (total 1.5 Gb), the computer has the same problem (starts for about 1 second, shuts down for about 2 seconds, starts up again on its own, but the BIOS screen never appears and Windows never starts). So, the problem seems to be that only Slot 1 will not allow the use of a 1 Gb memory module.

Over 2 days, I have tested many configurations. I have successfully reset the NVRAM in each case using multiple methods, including removal of the backup battery, removal and installation of the CMOS jumper on the motherboard, and resetting to the Default config in the BIOS. The RAM memory installed is easily recognized in every case, except when a 1 Gb Module is installed in Slot 1. I have two 1 Gb modules, and both work with no problems when installed in Slot 2 with nothing in Slot 1 or with the 512 Mb module in Slot 1, but nothing works when a 1 Gb module is in Slot 1.   

I bought my Dimension 2400 new in 2005, Celeron 2.4 Ghz, Windows XP Service Pack 3, Bios Rev A05, with 512 Mb (2 x 256) of RAM.  Upgraded to 1 Gb (2 x 512 Mb) a few years ago, and recently bought two 1 Gb modules to upgrade to a total of 2 Gb.  New modules are 1 Gb  PC2700  333 Mhz  DDR  Low Density. 

Any ideas as to why a 1 Gb module in Slot 1 only would prevent operation of the computer?

The default is that I have to live with 1.5 Gb of RAM instead of a full 2 Gb. 

Thanks