Edited by: Thommy on 04/10/2006 10:47:02
It's possible by a great varity of ways an virus can survive.
The next ways are possible (some allready mentioned):
- Backup's infected
- Hidden virus (master boot sector, motherboard bios even some nasty ones that can introduce themselves to other hardware biosses like flashable dvd burners)
- External infection by vulnerability (operating system weaknesses)
- Chain virus / root kit (an virus which has split itself into an multiversity of files / chunks and can be resideing anywhere in many locations on your computer mostly stealthing itself)
- Infected installation software (slipstreamed windows OS cd's, driver disks etc)
To remove everything is going to be though especially in the case of rootkits since not all are detectable without alot of "firepower".
The best way to do it is to do a 1 by 1 cleaning (which basicly means disconnecting your computer from network) and pulling out all unneded hardware fr boot and use the original windows disk only (since that should be secure).
The steps are very long ones and involves a 1 step at a time cleaning process to be absolutely sure (insert each and every hardware part only 1 at a time).
You will need an very good virus scanner aswell as other specialised programs which can properly check flash biosses and sectors on your harddisk (its possible to low level format the harddisk but its very risky as its a 1 way process, anything goes wrong you MAY loose that harddisk!).
NOTE since the computer MAY be infected you will need other ways of getting the files trough uninfected (ordering of disks, downloading at an friend with a clean system, going to a computer store WITH good support etc)
To better protect in the future its recommended to get an router with port shielding (for example the next device).Router
Attention though this is only a router and NOT an cable / dsl / sat-tel or any other wired / unwired external connection device (modem).
This way your computer(s) are less vulnerable to outside attack but you will still need other protection tools though since this is not an catch all tool.
EDIT: added note about getting files trough uninfected which is crucial *duh*