So far Avast 5 been a pain in the a$$ for me. I've even posted my displeasure on the Avast! forum, where there are many more people having lots of teething problems.
Since "upgrading" to Avast 5 on January 21...
1. I had to uninstall and reinstall Java. Twice.
2. I had to reinstall my chipset drivers after SATA I throughput fell from ±130MB/sec to just 4MB/sec.
3. I've gotten 0x80040507 bootup errors three times saying that I should break out my XP Home disc and do a repair installation. Clicking "OK" on the dialog box brought up the password field as normal anyway. Go figure.
4. I'm getting "ESENT" errors in Event Viewer every few minutes:

5. I would say that the A-ball systray icon appears automatically only half the time. The other half, I have to open Avast and click "OK" on the Settings page.
I am this $%#@ close to rolling back to version 4.8. I hate version 5, and I'm not alone.