Whilst I have no idea of any compatibility issues I'm not sure how much of an OS 'Windows XP media edition' is. However, it is not on the supported OSes list, if this is just XP with extra functionality to support media, then perhaps it might install but it would obviously have an element of risk.http://www.avast.com/eng/avast_4_home.html
* Windows 95
* Windows 98
* Windows Me
* Windows NT 4
* Windows 2000
* Windows XP
If you had a means of restoring your system pre avast install, like a drive imaging tool, Drive Image or Ghost then you could probably try it and see.
The safer bet would be an answer from one of the Alwil Team.