If there is no entry in add remove programs for avast! then you have a corrupt installation and it wouldn’t matter if you were in safe mode as the entry if missing in normal mode would also be missing in safe mode.

You need to download, save and use the avast! uninstall utility from safe mode as mentioned above.

Getting into safe mode is normally done by clicking F8 (keep pecking away at it) during boot and that should give the option to boot into safe mode.