Avast deletes tcpip.sys on Windows XP with update 121205-1 if the file TCPIP.SYS has been patched with Lvllord’s TPCIP patcher. This is a Patcher to increase the limit of concurrent half-open connections. The patch can be found here http://www.lvllord.de/
Can confirm this on multiple Windows XP systems. Please fix the avast update