Same problem on same MOBO
I had exactly the same problem with 'Fan speed too low' warning on the same ASUS mobo. I have had it once this week - no new hardware installed running XP Home.
The fan was running but slower than usual - I have the same default warning speed.
I checked two mins later and the speed was normal. I disabled QFan support and have not had problem since then. I am unsure if there is a connection with QFan but I do not want a burned out San Diego 3700!
I also have had a driver/ GFX card problem see my other thread:
http://forums.hexus.net/showthread.php?t=65313
This has all happened very recently and my machine does not always shut down. It always shows a process running called 'remote.exe'.
I am wondering if the MOBO is playing up at a chipset level.