Well.....
If your g/f really has a virus, it may not let her install any anti-virus software. I have seen this situation several times before and haven't had much luck in getting anti-virus/spyware scans to run locally. I'd say you basically have 5 options:
Option 1. You can pull the hard drive out of her computer and intall it in another system that already has anti-virus and anti-spyware software installed and do a deep scans on her drive. This might get you to a point where you can install anti-virus etc on her system. (You would, of course, have to put her drive back in her system)
Option 2. Definitely more involved but once you do it the first time you then have an excellent boot CD for emergencies like this. Try making your own Bart's PE CD.
http://www.nu2.nu/pebuilder/
You'll need a couple of plugins, namely the Lavasoft plugin and to download AdAware.
The AdAware Plugin:
http://www.nu2.nu/pebuilder/pluginhelp/adawarese.htm
AdAware Program:
http://www.lavasoft.de/software/adaware/
For anti-virus you'll need these 2 plugins:
http://www.pe-plugins.ncpp.de/download.htm?AVPersonal7.zip
http://www.pe-plugins.ncpp.de/download.htm?CWLoader_eng.zip
And this free anti-virus package:
http://www.free-av.com/down/windows/antivir_workstation_win7u_en_h.exe
Takes some work but it's definitely worth it.
Option 3. Use the restore disks that shipped with her computer to restore it to "Factory New" condition. Assuming, of course, that she has restore disks. This will wipe out anything she has added or done to computer since she bought it.
Option 4. Wipe the drive by reformatting and then re-install Windows
Option 5. Yell, curse, scream then toss computer in the garbage and buy a new one.