For security reasons, avast! will not start if it finds an active virus in memory during its startup.
We are currently working on a standalone tool that will remove known worms from your computer. This tool will hopefully become a part of avast! in the next major release. While it's still under development and testing, it is capable of removing Opas worm already - so, if you want to try (it's quite safe I'd say

), I can send it to you by email - it's about 100kB.
Otherwise, there are other tools for removing Opas worm from your computer. Or - when you check the information on the virus, e.g. on our web pages - it's not particulary hard to remove the worm by hand.
ig