I've built a few rigs in the past and never had any problems until now. This comp will shut itself off during various phases of startup... either before windows boots or sometimes after the desktop is mostly loaded already. Sometimes I also get an "overclocking error" but when I check bios it all seems to be on auto/standard so I'm not sure why it believes to be overclocking. My guess is overheating but the heatsink seems on good and I used thermal paste so I don't know... tried with case side open and fan blowing on as well. Temp gauges read low 30sF so seems normal. This is an Intel Pentium D 805+ and Asus P5LD2 Mobo... Aspire case came with 500w P/S. Thanks for tips fellas.