Coming off of the Acer Aspire One review, I was baffled at the inability to control many of the hardware components in the interest of power-saving. Maybe a remedy to a small part of that is the Aspire One fan control utility, which is a freeware application written by Ralf Neumüller.
Just download the tiny file (right around 700kb), unzip, run the installer file, then launch using the AA1 Fan Control icon within the same folder. Once you run the installer, you can put the icon anywhere for easy to reach access. You’ll need to do a restart after the installer runs. Once you boot back up you can launch the fan control utility and you will be greeted with a clean little bar that pops out of the task bar and will allow you to change the temp threshold for when the fan turns on/off. You can also see the current temperature of the CPU.
A quick install on the Aspire One that is heading out soon and I was able to confirm that it works on XP as well as Vista (judging by the image on the AA1 Fan Control download page). It looks like it has also been recently updated to work with all Aspire One BIOS. Just make sure you know what you are doing when changing the fan thresholds so you don’t end up frying your Aspire One!