Built a new PC, installed Vista and a few other apps, all was well for the first 20 or so boots.  Then, randomly during POST, PC powers off.
 
Does this sound like a bad PS to anyone?
 
Thanks

Possibly but more details needed. Post your system specs and temps. Could be that CPU is overheating quickly and it is automatically shutting down for safety.

Thanks for the reply,
 
System is as follows:
MB: GIGABYTE GA-P35-DS3L LGA 775
CPU: Intel Core 2 Duo E4500 Allendale 2.2GHz LGA 775 65W Dual-Core Processor Model BX80557E4500
VC: XFX PVT84JUDD3 GeForce 8600GT XXX 256MB 128-bit GDDR3 PCI Express x16
HD: Western Digital Caviar SE WD1600AAJS 160GB 7200 RPM SATA 3.0Gb/s
Mem: CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
OS: Microsoft Windows Vista 32-Bit Home Premium
 
I don't think it is a heat related problem because it is reproducible from a cold start.

Disconnect the power switch from the mobo, and short the pins with a screwdriver.  If the behavior stops, inspect the power switch and/or wiring; this behavior could indicate a short.  Your PC shuts off just as though you'd held the switch for 4 seconds.  If it doesn't it could be the PSU or mobo.  Get a PSU tester and see what that says.

Agreed I was just about to say something along those lines. What PSU are you using?

Thermaltake Purepower W0100RU ATX 12V 2.0 500W Power Supply
 
Thanks,

That definitly looks like one of their lower end units. I'd suggest you check the UL code on the side and see who makes it. The gradual degradation sounds as if it could be power related.

Thanks to all of you who tried to help.  As it turns out, the case and components were manhandled by UPS which caused a number of plugs to come loose.  All is well now that everything is attached.
 
Regards

Hah, I've even had UPS delivery throw a box of memory at me.