It's a bit limited as it's only designed to deal with stuff that's currently doing the rounds, but stinger from McAfee is small and free ...
http://vil.nai.com/vil/stinger (worryingly I know that url by heart)
Failing that, if you know the names of the little critters try putting the name followed by 'removal tool' into google as some of the trickier ones end up with their own special removal tool. It's also worth looking in the McAfee library (
http://vil.nai.com) for details of particular viruses, although a lot of the time it can get down to manual registry editing so I wouldn't recommend it unless you know what you're doing as registry changes are INSTANT and IRREVERSIBLE (
so create a restore point before you try, just in case).
You could also try running a scan with Hijack This (
http://free.antivirus.com/hijackthis/), but again, be aware that this is quite a powerful tool and IRREVERSIBLE DAMAGE may result if you aren't careful using it.
And yes, I am being a bit of a nanny about some of these tools/techniques. I've seen very experienced professionals make a teeny mistake and completely knacker a machine.
It's also worth noting that some viruses have a hidden component that sits and basically keeps checking that the 'virus' file is still there and running. Every time a virus scanner deletes it the hidden bit simply recreates the file and you're infected all over again. If you find this happening, a handy tool to have is Process Monitor (
http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx). This is an updated version of the old sysinternals filemon combined with regmon. Among other things it can tell you what files are calling/accessing what other files, registry entries, etc so you can find the hidden 'parent' file and kill it.