Adding RAM causes problems

Recently I purchased a 1Gb dimm module for my PC.
I REPLACED the existing TWO simm modules of 256MB each with the 1Gb ram.
The PVC operated ok.

Today I decided to ADD the two modules to give me a total of 1.5Gb.

The user's manual for the motherboard (Gigabyte GA-8PE800 Pro) states that there are 3 dual inline memory module (dimm) sockets. and that the motherboard can support up to 2GB of memory and that DDR1 can support a double sided memory module and the other 2 sockets each a single sided module.

When I started up my PC the POST displayed a checked 1.5GB ram HOWEVER when Windows attempted to start it causes the PC to REBOOT.
When I removed the 2 single sided ram modules Windows starts ok.
Any idea why Windows refuses to work with 1.5GB of ram is installed?

 

I'm sure the dimm modules are in the correct slots.
The manual says that slot 0 can take double or single sided ram (that is where I have the double sided 1Gb module) and slots 1 and 2 can take single sided modules when slot 0 has a double sided module (that is where I have the two single sided 256MB modules).
Besides the PC boots up past the POST successfully (usually it will not do this if there are memory problems - right?) and begins to start Windows so it seems to me that Windows is complaining and that is why I posted in the Windows (XP) forum.
I have also tried the double sided 1GB in slot 0 and a SINGLE 256Mb single sided module in slot 1 and still it keeps rebooting as Windows starts to load.
Also all dimm modules are the same type (333 mhz)
I am sure it's got something to do with Windows what what is it?