As you all know virus is always being a annonying part of computer system.virus creates problem in proper management of pc running,hardware/software/network problems.even the real fact of matter is that antivirus companies are making viruses and then spreading them & also giving their solutions at the same time.according to the working & behaviour of virus they are categorized into multiple types like boot sector,files system,replication/duplication,registry etc. etc. virus.in actual a virus is a software/aaplication program can be written in C,C++,VB,Assembly,Python etc.depend on programmer a virus can be just havocing a system or as much as destructive having destroy of all data on network & across network. as far as my studies say we can remove a virus without help of antivirus programme,again it depends according to behave & type of how virus is working.some common methods are- 1.some virus disables your registry,msconfig,system management,folder options(for hide files),services.msc,gpedit.msc(NOS),cmd so what should we do now,as soon as we execute these cmds system gets hang ot gets reboot(force stopping of shutdown now work here).goto startup entry of start menu see there any annonymous entry is working or not,if working simply delete it.most of virus load while startup of windows.they attach the entry in boot.ini,autoexec.bat etc.if registry is not working open win.ini & find any entry name as load= & run= this must be blank.if any anonymous entry is there deletd it & save.again one more thing if registry is working naviagte registry & find this key- my computer\HKEY_CURRENT_USER\softwares\microsoft\win dows\currentversion\run in the right pane find all mallicious entry these entry can be examine if you are having good expose of background processes of of windows.next step is to identify those nasty things is that look for some weird chars or symbols associated with that file(may be multiple ? marks or some special chars or something like win 32,.vbs.exe/any 2 extension dll or exe)continue.... delete that entry from right pane & simply refresh registry using F5 so the registry setting will be applied at that time(remember don't mess with registry untill you don't know how to work in registry else your system may gray out & crash too)close the registry & come out.now next step go to task manager & goto process tab.now here once again you have to identify a malicious script or exe running,it may have something horrible name or 2 ext. name or something pointing to unknown directory,kill that process(at least we can kill someone without afraiding with police LOL)ok back to work.now initial stages we have completed but this not means we have removed virus.now go to windows temp directory & clean it,go to temporary interntet files & remove all,delete cookies too.here we have completed all major steps.better you do this all tasks in safe mode.restart your system to take changes effect.if its not a sticky **** piece virus it will remove & if it not in advanced stages I will guide how to remove.keep in mind the posible reason when you can come to know about virus attack- 1.pc slow 2.process taking lot more memory space 3.startup/shutdown taking a lot time 4.CPU utilization is 100% without opening multiple appz. 5.I/O cycle is slow 6.hanging 7.crashing 8.options got hide 9.no network drive its showing 10.less space showing then actual disk space 11.any pop-up/add/banner is coming 12.one file duplication in all over folders. 13.duplicate files having same filesize 14.some .exe files in face of folders,music,jpg,bmp,pdf,doc,fla,xls etc. don't run these kind of files. 15.large size of caching is done & PF usage is high. 16.frequently system files getting replaced or delete. likewise so many reasons are there.its you have to find what is abnormal.these are basic practice of how we can remove a virus.now surely in your mind a clear picture how virus is actually working.