Did you check first to see if your system was compatible with Grimefighter before installing it? info can be found here
http://www.avast.com/en-gb/faq.php?article=AVKB120#artTitleIf you meet all the basic system requirements and avast! GrimeFighter is still not compatible with your system, this can be attributed to the following issues:
•avast! GrimeFighter does not run on any Apple Mac OS, or any other system which isn't specified as supported.
•avast! GrimeFighter is not supported by all graphics cards.
•avast! GrimeFighter does not support RAID. If a RAID controller is detected currently, avast! GrimeFighter does not run as a precaution to avoid breaking any RAID striping, potentially resulting in a loss of data.
•avast! GrimeFighter requires a constant Internet connection. If you’re having connection issues, try connecting to your router with an Ethernet cable for a more stable connection.
•Support for Windows 8 / 8.1 is on our short term roadmap as we find a way around the system’s default encrypted disk setting which prevents us from successfully scanning your computer.