All I can suggest is that the MS Antispyware Real-time protection has added the avast! antivirus services to its blocked list and has now prevented Windows from detecting avast.

Open MS Antispyware, then click on ‘Real-time Protection’ under the System Summary heading. Then click on ‘System Agents’. Select ‘Windows Services’ and then click on ‘Manage allowed/blocked WindowsServices’ under the details section. From the drop-down box, select ‘Blocked WindowsServices’ and see if avast is there. If it is, remove it from the blocked list and hopefully windows will detect avast again.