for future people from google search results, i just had the same problem too
the sequence i had was:
1. installed avast for exchange
2. realized i hated its spam filter, so disabled that "shield"
3. realized i hated the yellow triangle forever over the avast icon warning that "not fully protected" because antispam is turned off
4. in control panel, clicked CHANGE to remove the antispam portion of avast, rebooted the exchange server
5. at that time the exchange transport agent for avast antispam was still trying to start, since it didnt, it crashed the mexchange transport service
6. we chose to reinstall avast, which fixed it, but brought back the antispam "shield" (and associated file which was missing)
7. remove the agent from exchange command line: disable-transportagent "avast antispam for exchange"
as soon as there are more antivirus providers for exchange 2010, we'll go there instead... but so far avast and trend micro are the only ones and avast sucks less
BY THE WAY - if your avast subscription runs out, your information store WILL NOT START, errors out with "failed to start virus scan"
i had 29 days left, reinstalled, now zero days left. fixed the agent but now information store wont start.
avast, please at least try to beta test before releasing this stuff... please?