It would be handy if you had mentioned the error you are actually getting ?
If you have a corrupt installation then the uninstall as you find may not work.
Try a clean reinstall:
- Download the latest AIS version of avast 6.0.1125 and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear.exe
find it here and save it to your HDD (it has uninstall tools for both 5.x and 6.0.x).
- 1. Now uninstall avast! (using add remove programs, if you can't do that as you mention, start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 5.x if previously installed and then for 6.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.