Well there are several people on the forums using avast on Vista 64bit bit XP Pro 64bit on the forums and nothing reported in the forums on the problem you describe.
What other security software do you have installed ?
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
A factory restore could well be putting on an OEM AV trial, etc. back on your system.