Exactly what memory did you buy, and what's your motherboard model? Links to newegg/wherever would be nice. It sounds like voltage/timings aren't correct when Windows is trying to copy OS files from the disk to memory. Some memory needs to run at higher voltage (2.0-2.2V from stock 1.8V), and it'll flake out at some point after POST, but during operation. Having aggressive timings at stock voltage can also cause this problem.. so provide the details and we'll try to figure it out.