Question about Computer Rebooting?

Built a new Pc, everything wooks fine, execpt when I go thru normal shut down , after it shuts down it automatically starts right back up again like if i selected restart instead of shut down. Temp is only around 31C
I bought a cheap no name 550W Power supply, could this be the problem or the wiring?

Specs are
Amd64 3000+
Giga-Byte Ga-K8NNXP -mobo
Giga-Byte MX4000 128MB Video Card
Geil 1GB PC3200 Ram
2 WD SATA 300GB HD Raid 0
Win XP

 

You don't really need a 550W psu in a system like that, a 450W would be fine.
Yes try another psu, better brand name and also check all drivers are installed correctly.