Sounds like there is some corruption of the installation or possibly something more serious, but lets try to deal with the first point first.
What errors are displayed, with these can't dos ?
First - Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
If that doesn't work try, uninstall (see below), reboot, install, reboot.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs or move to step 2 if you can't), reboot
2. run the avast! Uninstall Utility, reboot.
3. install the latest version, reboot.
It would probably be best to first Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Second, possible infection:
If you haven't already got this software (freeware), download, install, update and run it, preferably in safe mode and report the findings (it should product a log file).
1.
SUPERantispyware On-Demand only in free version. Or
Spyware Terminator Resident scanner (if you use this don't install the toolbar or crawler or the anti-virus module). Or
a-Squared free. I suggest trying then in order as the order that represents the better detection and clean-up. Some elements of the programs might not work if you have an older OS like win9x or winME, this is namely the resident protection in SpywareTerminator.
