Pc keeps restarting/won’t switch on

I have a hardware problem and would like to know if I have damaged my motherboard, whenever I try to switch my pc off it keeps restarting (windows closes down easily) and when I have switched it off at the psu switch it won’t power up (motherboard lights up, acts like it’s dead otherwise) until I tilt the pc over. I remember the white plastic clip bit of the pci express slot breaking off when I tried to fit a my nvidia 8800 gts graphics card, would it be because of this or the power supply?
I have an Antec 550w psu, an Abit motherboard with 2gb of DDR memory and a Athlon 3700 cpu running Vista Ultimate, I have no obvious problems with Vista.
Thanks for any help.