I use Avast typically, but there is malware out there for pretty much any version of Windows that can get around most scanners. I also use SpyBot and to a lesser degree, other spyware scanners, and they help, but after more than 20 years in studying and dealing with viruses, it's still an ordeal to get rid of some of the bad code.

Usually what I end up having to do is boot into Safe Mode a few times, running SpyBot to clear what it can find (much of this malware will hide and reload itself upon reboots or loading of certain software, especially web browsers) and going through the system, file by file, and line by line in the registry. Needless to say, this method, while rather effective, gets time consuming enough that after a while it's just easier to wipe and reinstall everything. I tend to stick to it partially because I know a lot of tricks to getting after these nasties, and partially because I don't like to admit to failure.

If you can back up vital stuff and reinstall, maybe with a new OS if you feel up to it, I'd recommend it. It would be time consuming and difficult to get into some of the more complex malware removals, and there's a good chance that even if you have all the newest security patches, some of that software will still find its way back in time. Running Linux or one of the BSDs virtually guarantees that you will have little if any trouble again, and you can download and boot off of live CDs/DVDs so as to try them before actually committing to an installation.