Good PC bad PC... help fixing blue screen cold boot

I have two computers based on MSI P45 Platinum/P45 Zilent. Both have updated bios to 1.6 which is the latest. Both machine built about the same time. The first build has always been problematic even at stock/default and the second is fine and holds a reasonable overclock. PC1 (the problem build) has two hard drives. XP is installed on drive 1 and Win7-64 on drive 2. Switching between the OS is by changing the boot order in the Bios. Tbh this isn't really relevant other than to say XP installed easily and windows 7 took 3-4 attempts to install. PC2 (the good one) has a single hard drive running win7-64 and never had any problems installing.

I'm trying to fix PC1 which double boots sometimes with a blue screen as the windows 7 starting logo first appears. Restarts seem to be OK. Just boot from cold is bad.

I have swapped the hard drive between machines since the builds are based on identical mainboards. My perfectly good windows 7 64bit from PC2 now inherits the same problems in PC1. I take this to mean it's hardware and not a corrupted windows installation.

RAM doesn't seem to be the problem. Tried different RAM several times and voltages 1.8v to 2.1v depending on the sticks used. Also tried 2 stick vs 4 sticks. This hasn't fixed the problem so I'm going to rule out the RAM.

I have just swapped the GPU between machines. Problem exists with GTS250 and ATI HD 4890. I'm not sure why I suspected the GPU.

I suspect I have a faulty motherboard on PC1. I've never had a dodgy intel CPU and the PSU is a decent 500W Seasonic based unit.

I got the boards from scan at a bargain price of £54 but faulty is faulty so maybe I can get it replaced.

Any good ideas to trouble shoot this one or should I be looking to RMA.