If you Google >free online virus scanner< you will find all sorts of helpful scanning sites.
http://www.google.ca/search?q=free+online+virus+scanner
If the scanner identifies a virus-infected file, you'll have to replace it somehow by finding your Windows Install CD / DVD, or finding and downloading a legit copy of it from the Internet.
I tend to use a couple of scanners to double check my system.
I used to use Trend Housecall, but they have made it a bit more complicated to use now.
I've also used Kaspersky, Panda Active Scan 2.0, McAfee, and so on.
Once the virus infected files have been identified, you can use Search for Files in My Computer by Right-mouse clicking the C-disk drive, select search, enter into the search criteria for the name of the file to be the name of the virus-infected file. If your search turns up lots of copies, you might have to reboot up into safe mode to delete / replace each one with a known good version.
The Windows operating system uses a protection system that keeps a backup copy of each important driver / DLL file that might mean that a duplicate virus infected file is being kept hidden so that it will overwrite the good copy with the virus copy again, so you have to replace each and every hidden or visible copies of the files with good ones. If the virus infected files are not .dll, .exe, .ocx, then usually they are safe to dump them into the recycle bin because they will not adversely affect your system's operation (because anything else will not be an executable system file). You can usually delete stuff like .eml, .bat, .com, .vbs, .vba files. If all else you can investigate various "file types" via Google searching for the dot extensions and reading about them before you do anything that you might regret later.
http://www.google.ca/search?q=apt+advanced+process+termination
Sometimes you can use the task manager to see what strange processes might be running on your PC, but other times, the virus prevents the task manager from running. In this case you need to run a program like apt.exe that has a different name than task manager that can list and terminate processes that appear not to be legit windows functions. Apt is just a utility that kills tasks via GUI selection of the running tasks. If a task won't terminate, then it might be running as a system service. You have to check with Google if a given process / program / etc is legit. If it is a virus in a service, you might be able to do Start-Settings-Control Panel-Administrative Tools-Services, then look through each service that is running to try to find the virus one, then click on stop service, then using properties of the service, select Disabled instead of manual or automatic. Some of this might have to happen in Safe with Networking mode.
In 1999 I had troubles with a virus that kept launching itself. I had to find all of the unusual processes that were running and terminate them, and then to examine the registry to find out how they were getting launched in the first place. This usually happens from within the system registry in the keys similar to MyComputer-HKLM- Software-Microsoft- Windows-Current Version-Run registry node. A similar node exists under HKCU, and so on.
Obviously there are legit startup programs there and may also be viruses too. To disable a file from running, I simpley open the string variable, and change it to have a leading minus sign. The key will remain in existance, but when Windows tries to run the program mentioned in the string, it will get a file not found error and continue without running that item.
MSconfig.exe is also available to examine startup time software and be able to disable it from running during the next reboots. http://www.google.ca/search?q=site%3Awww.microsoft.com+download+msconfig.exe
Any software that you run by viewing / previewing an email, or running some software can lead to virus infections. Before doing this kind of activating / viewing / previewing of anything mysterious, you can upload / send the suspicious file to Virus Total for analysis where the file in question can be scrutinized by about 30 to 50 different virus scanner software packages of very recent vintage.
http://www.google.ca/search?q=%22Virustotal%22
You can set most email packages to not preview any emails, in that way preventing infections from happening from javascript or Visual Basic for Applications script from injecting the virus into your Windows PC.
Sometimes it is best to have your windows system disk, backup all of your emails, photos, and data / documents, reformat and reinstall windows to get rid of the virus and take note of what you are re-installing, step by step by writing into a .txt notepad file located on removable media such as a diskette or RW optical disk. If everything works great up until a particular step, then you'll know that something suspicious during that step or the one prior has caused the troubl