if there is a active virus running u can download process explorer…
http://www.snapfiles.com/get/processexplorer.html
a advanced version of the taskmanager and find the files from which the virus is starting up…
and kill the process…and remove the files.
if u plan to use avast…there is a option to scan all ur system files during startup
[ie;BOOT-TIME SCAN]
including “system volume restore information”.folder…from where the malware may infect again…

Name : Brontok.N
Alias: WORM_RONTKBR, W32/Rontokbro, W32.Rontokbro.X@mm, Email-Worm:W32/Brontok.N, Email-Worm.Win32.Brontok.n
Type: Email-Worm
Category: Malware
Platform: W32
Radar

CHECK FOR THESE PROCESSES
csrss.exe
inetinfo.exe
lsass.exe
services.exe
smss.exe
winlogon.exe
Some of the worm’s files have hidden, system, and read-only attributes. The worm can create its files with COM, EXE, and PIF extensions. Brontok worm creates multiple launch points for the copied files. Those include startup Registry keys as well as scheduled jobs

source>>>>
http://www.f-secure.com/v-descs/brontok_n.shtml