every 2 days avast server is uninstalled

Avast Business Avast Server Protection
1

hi,

every 2 days i login to my server and the avast icon has has a no entry sign next to it
and the program is uninstalled.

i reinstall, update and do a boot time scan - all seems fine

2 days later the same thing happens

win2003 web edition service pack2

any ideas welcome

MG

2

hi - just done another boot time scan and the following are infected

win32:trojan-gen
c:windows\system32\dhcp\dhcpsv.exe

win32:adware-gen
c:\windows\system32\dhcp\winlogon.exe

win32:adware-gen
c:\windows\system32\dhcp\temp02

restarted and all seems fine - any ideas to stop this happening?