is it a windows blue screen? Have you tried booting up in safe mode? Also after installing this new hardware, have you gone into the Bios and set system to Optimized/remove the battery on the motherboard for 5 minutes or so, so system is reset to factory defaults? I would say if the pc is booting it is fine, maybe a memory module is duff, I asume that you are running more than one dimm. Try removing one/alternating the setup ie process of eliviation. Maybe someone else can give some input, but should the PC not work after resetting BIOS, I would put this down to a dodgy memory stick.
Let us know how you get on