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Just to keep everything in perspective, the more a programs evolves (like antiviruses through updates) the more likely system restore will bonk the program. In the past year, I used Avira before avast, and I can tell you system restore would corrupt that installation especially if the system restore point was older and there was a revision to the virus engine in between.
In regards to Avast, I have successfully system restored many times but only with manual restore points less than a day old, so it can be done. Three times just in the last month in fact, b/c I was switching out various network cards to diagnose a connection problem.
It is unrealistic to expect all software installation to remain unscathed from system restore. In general, it is a bad idea to use system restore for anything other than a very recent point. In fact I have seen very well known and expertly written software like Google Chrome become corrupt from system restore.
Really just uninstall the program, and reinstall Avast and move on. The thread moving from legitimate vent, to something else all together.