what would you do if your computer is infected with virus?  

My friend is called Mr. Format. :D If antivirus can not do anything, IMHO you have no other choice than backup the most important data and make format. Or maybe you can use system restoration point, but I have not tried it. Good luck!

When ever system is infected I'll take backup,run McAfee antivirus in safe mode.Then run NoAdware. Or you can take Ghost image and restore it when ever there is problem.

First of all Have a good antivirus (I use Avast antivirus with windows Vista,And believe me there is no need for anything else)you can make ghost of your windows when it is not infected & all setups are installed.When computer become infected run your ghost & get your window with all setups installed in just few minutes.Also You can install all setups in Local disk "D" instead of "C". Some setups may Deny but other will work.

In this context I do remember a famous saying:
"Prevention is better than cure"
Once our systems got attacked by a virus we have very little things to do, but certainly there are:
1)Run any good antivirus installed in your system and scan your PC completely. It will atleast tell the name of the virus and the areas affected by it(atleast the drive) but will not delete the virus files if it has attacked the system files.
2)Then search for virus removal tools for that particular virus on internet and using that tool check your drive infected by virus atleast twice.
3)It will delete(if possible) the main root cause file of the virus.After this we can again run the antivrus program installed and try to scan the PC.
4) If again antivirus is unable to delete the infected files, it will give the names and location of the files. Use this information to delete the files from command prompt in administrative privilage(for that type cmd in run prompt and instead of just enter press ctrl+shift+enter)
5) If still problem persists or virus has attacked main system files which prevent windows to start then try all the above steps in command prompt with safe mode(press F8 before bootin, it will open a menu and choose safe mode with command prompt)
6)Now all the viruses have been removed from your system, but if now you have deleted some important system files then you have not to format your system rather just try to repair the system from the CD. It will simply check for missing windows files and copy them OR after removing all files if windows is able to load normally then try to restore your system using system restore option available in control panel(restore system to a date before being attacked by virus)

Some prevention methods:
1)Always use a good antivirus software( like Bit Defender, Quick Heal, McAfee etc) and update the definitions regularly.
2)Make a ghost image of your main root drive when your system is clean and use it in such situations.
3)Make atleast 2 partitons of your system, one conatining the windows(root drive), other containing your data and all.
4) Always store your data in drive other than root drive. In root drive try to only store installed programs and other things which require direct connection with Internet for updates etc. This will help you in case if u want to format the system, as now dont format the whole system rather only the root drive and now no personal data will be deleted just the installed programs will be deleted which can easily be installed again as you have setups of them in your second drive.

I think these points will help you definitely.

always update your antivirus.
i prefer avira .. it has a free version. If your antivirus fails then you can have your PC scanned online. try bitdefender.
after your PC is clean but still not working properly then your system files have been affected. for this you will have to check your system files.
go to - start - run
in the window that opens next type -
sfc /scannow
make sure there is a space after sfc
this will check your system files and if any file is corrupt it will be repaired.
you may need your original OS installation CD/DVD
insert it if any message for same appears.
hope this helps
good luck

Nowdays.....just antivirus software won't cut it...you need good softwares such as antivirus, antispyware, and a firewall to make sure your computer is safe. Better yet, these antivirus, malware and firewall companies are offering all the softwares i just mentioned in one......they're called internet security softwares. This is your best bet if you want to have piece of mind with your computer. NIS(Norton Internet Security) 2009 is a really good one and the KIS (Kaspersky Internet Security)too. They will cost you average 30 bucks but it will be worth it...don't go for the free softwares such as avast, avira, and avg...i've used them for about 3 years and problems problems problems!

Perhaps you can try Avira-Anti Virus and use ZoneAlarm to setup a fire wall too

in my experience, i do not get too many viruses, however when i do it is usually time for a format anyway because of windows f*****g registry slowing down my PC, so i back things up and format. i never usually have time to build up a lot of data on my hard drive between formats. and i have a 1tb external on which i keep all my media files.