I started having this problem as well. Played ME2, ME3 for hours with no problem. Played a few minutes of GW2 in Windowed mode for about fifteen minutes and bam shuts down. Specs below:
Win 7 64 bit pro
8 GB DDR Ram
AMD Athlon-6100 6-Core Processor
EVGA GTX 550Ti video card
1.5 TB HDD