What program(s) do you normally use for extracting .7z and other compressed files?
Actually, don't answer that, just go get 7-zip, it's open source and free and should extract your .7z files plus pretty much any other compressed format.
http://www.7-zip.org/
If you are relying on the extracting software that is included in some versions of Windows, it won't work with everything. I had to put 7-zip on my work computer last week because someone sent me a bunch of documents compressed into some format that Windows had no clue what to do with.