I'd personally run some good antivirus or antimalware rescue disk, like say Avira's or kaspersky's

If for some reason what gregrocket said fails and what I told above fails as well you can use any good linux live-cd (like say Ubuntu) to replace explorer.exe from one you pilfered from a working Win7 installation on another machine with that same disk and a usb pendrive. Practice using it a bit on less-critical files, as the interface and commands may not be exactly the same as windows 7.