Heylo all

I have just RMA'd my gf4 ti4200, and have a gf4 mx440 instead. I installed it, noticed the power light (on the PSU) was still on and flickering, turned it off.

Turned it on (so power is in computer), and turned the computer on. THe problem comes when i have to turn the power off and restart. I shut down as normal, but it wont turn back on until i have flicked the PSU switch... anyone know what it going on?