Hi sorefingers, using a Google search I found this at click here.
I have had exact same problem in the last few days. I had the nachi worm which AVG got rid of, then got the same dialogue box as you when the PC is on for a while. AVG doesent find it because it is in the system restore folder which has limited access. Your system has backed up when the worm was present. I have read you should turn off system restore (start/my computer/system restore/check box), restart then run anti virus again.
For full information on how to remove it, look here click here