how did you do it… did you use a factory CD/ HD partition and sett it back to factory specc?
or did you use a windows CD?

if you sett it back to factory specc… does that mean you also recovered a factory installed trail AV like Norton / McAfee / TrendMicro ?

if so, this must be uninstalled before installing avast
when done, run the vendors removal tool to clear any leftover files that may conflict.
tools found here. http://singularlabs.com/uninstallers/security-software/