I don't use Windows much anymore, but I've grown disdain for Avast and AVG, because of how much they've turned into nag-ware. In the virtual machines I have of Windows (and my gaming partition), I generally put Microsoft Security Essentials on them to keep Windows from yelling at me about not having anything. I haven't used it on a full-time Windows machine (and I still question the wisdom of using AV software from a company that wrote operating systems in such a way that requires AV to begin with), but for the limited amount that I use them, it's worked for me. For the most part, the best defense is to be smart about your Internet usage -- don't just download and install things willy-nilly, don't just blindly click popups, pay attention to where emails come from, etc.
GoofKing, as for your girlfriend's computer, it sounds like it may have already been infected, and it's probably not your doing. A lot of viruses are smart enough to try to protect themselves from anti-virus programs, especially if one hasn't yet been installed (in this case, changing the exe to a non-executable extension).
The Adobe and toolbar thing are probably just bundle software that Clamwin offers as part of a deal to get some funding to be able to continue to offer a free product. Just pay attention at install and make sure to uncheck the extra software.
I recommend installing
Malwarebytes Anti-malware (Mbam) to help remove anything that already exists. The free version suffices, and it doesn't really nag to upgrade (you just only get the reactive tools, instead of proactive). If you run into the same problem that you did with Clam, where it seems to download wonky or won't install, Malwarebytes has a "Chameleon" version that is supposed to be able to install on an infected computer. Ideally, you'll want to start Windows in safe mode (press and hold F8 as the computer starts booting into Windows and you should get an "advanced startup" screen, where you can select "Safe Mode with networking"), to limit what is running on the machine and allow Mbam to remove as much as possible. Let Mbam run a full, thorough scan and cleanup, then restart the computer, go back into safe mode and run it again to make sure it was able to get everything. Once you get the all clear from Mbam, restart the computer and boot into Windows as you normally would.
Once you're into Windows normally, remove the old anti-virus software (having two proactive anti-virus programs can cause more problems than it solves; you can leave Mbam, though, because it's reactive), and install whatever new title you want. Get it updated and let it do a scan to verify that you've cleaned up the computer. Clean up anything it finds. If it finds stuff, but can't clean them up, boot into Safe Mode again and run the AV again.
If it still can't clean everything up, then you should probably take it to someone who knows what they're doing to either manually root out the remains or completely wipe the machine and start over fresh. (If you want, I can walk you through the "start over fresh" route, should it come to that, it's actually pretty simple.)
In any case, I recommend backing up any data on the computer to an external location (and run a scan on that stuff, specifically, to ensure that it's not infected, so you don't infect anything else or re-infect your computer).
