Allocation errors are many times caused by hardware issues. Sometimes a faulty OS or a bad dirver is is the cause. I suggest you repair them, if you have not done so already, with a disk util like Norton Diskdoctor. Make sure you create backups of important data on a regular base, just to prevent you loosing it when it really goes wrong. HERE is a nice explanation on how HD’s work which you may want to read in the mean time. Keep an eye on the problem and if it keeps comming back, start a new thread in “off-topic” since this is not Avast or virus related.

Good luck.