Possible PSU problem

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Hi there,

I am having some troubles with my new computer which I think could be related to the PSU.

Basically when I shutdown my computer from Windows XP, it looks like everything shutdown (monitor off, keyboard lights off etc), but the power led and my case fans are still running. It isn't in standby mode, as I have tried bringing it out of that state. The only way to completely turn it off is by pressing the power button for a few seconds.

Here are my computer specs if they are required:

PSU: Antec TruePower 2.0 550W
Motherboard: ASUS A8N32-SLI Deluxe
CPU: AMD Athlon 4400 X2
HDD: Seagate SATA II 300gig
Memory: 2gig dual channel Geil RAM
Graphics Card: Gainward GS 512meg 7900GT
Case: Thermaltake Aguila

Any help would be greatly appreciated.

Thanks