I am trying to put a new computer together using a GA-M55plus MOBO.
Everything went fine until I did a shutdown from fresh install of XP. The computer shut down but the cpu fan (connected to the MOBO) and case fans (connect via a HDD 12/5 volt lead) kept spinning. I can't find anything on Gigabyte website and it still happens.
I can stop them by switching off the PSU or holding the on/off switch in for 4 secs.
Also if I don't use XP to shut down but just press the on/off switch in for 4 sec the computer and the fans shut down like they should.
Help please.

After a PC shutsdown the fan stops spinning and your cpu runs hot as hell only to be cool down by nothing <--- bad thing result in short lifespan
Possibly your motherboard's smart fan dection is wrong? Or fried?
There is a program that runs the fan on your mobo for 3 min after you shutdown to help cpu cool
Do you have that install?