hi tommysze,
Sorry to say this, but a Windows system file should never be deleted, even if Avast! says it is infected. Reason is, you cannot recover a deleted file of any kind, including a Windows file, once Avast! is told to delete it.
Options presented on detection and end of scan are four: Quarantine, Repair, Delete, Ignore. With a system file, best option is to use Ignore. With any other file, best option is to quarantine in case of a false positive report. Repair does not work at times because the file detected may not be actually be a virus, but a worm or Trojan. In the latter two cases, the entire file is the malicious agent; so there is nothing to repair, and Avast! Repair will fail.
Your options are always to be able to come here for expert assistance in such a case as this one. essexboy knows how to fix a real infected Windows system file without damaging your system; unfortunately, this C:\WINDOWS\system32\drivers\tcpip.sys file was deleted. He can still help you, tho.