More than a year ago, I bought avast! Internet Security for a year. From that time forward, my windows firewall was turned off so i didn’t care since avast! have its own firewall. Now, my Inter Security expired and im using the Free version. But i can’t turn on the windows firewall. I am using Vista x32. When I am trying to turn it on, it says “Windows cannot start the Windows Firewall service” Any help?
The avast firewall is designed to work with the windows firewall, so it doesn’t actually disable it. So there would appear to be something else going on on your system, e.g. why the service can’t be started “Windows cannot start the Windows Firewall service.”
Unfortunately I don’t use Vista so I can’t really say what to look for and I long ago gave up on the windows firewall, preferring to have a more user friendly one, which is more configurable and provides outbound firewall protection.
Lots of search hits on the windows vista error http://uk.search.yahoo.com/search?p=Vista+"Windows+cannot+start+the+Windows+Firewall+service", this one from MS seems a good place to start. http://answers.microsoft.com/en-us/windows/forum/windows_vista-performance/cannot-start-windows-firewall-for-vista/8d3ab846-d5a9-4df1-bb1e-a6f4842abc63
Microsoft has also released a Fix-It for that problem:
http://support.microsoft.com/kb/2271812
Hi
You can try to manually enable the service.
Open the start menu and search for “services” (provided that your system language is EN). A window will open, on that windows search through the entries for the “windows firewall” service. Once found it should be set to disabled. Set it to Automatic, and manually start the service.
Check now, and see if it is possible to enable the firewall.
Hope it helps 
EDIT: or just go for the link BOB posted, it should do this for you.
I suggest you go to Control Panel > Administrative Tools > Services and set the Windows Firewall to be automatically run at startup (if it is not already).
Unfortunatly, none of the above suggestions worked 
Sometimes, avast turns up a malware detection (that red thing) and says that there is a malware in “svchost.exe” and it deletes something. Does it have to do something with this?
Sometimes, avast turns up a malware detection (that red thing) and says that there is a malware in "svchost.exe" and it deletes something. Does it have to do something with this?i recomend you follow this guide, then a removal expert can check for any infections...
follow guide and attach the requested logs (not copy and paste) http://forum.avast.com/index.php?topic=53253.0
run in order listed
AdwCleaner / Malwarebytes / OTL / aswMBR
when done removal experts will be notified and check the logs for infections…
if trouble running any of the Tools, try run from safe mode…