Is there anything special about the Zip files you can't open? Where are they located? How have they been "handled" for storage, archiving, saving to external media? Have you used any unusual or new utilities on those files? Is this system an NT system? If so, what file system is it running? Have you used encryption in any way with respect to these files, or have you used it on the files that were included in the Zip files?
The most common issue causing the "not a valid archive file" error is a bad download. But if all of these files were opening okay until two days ago, and only files less than two days old can be opened now, it makes me think that something happened to corrupt them two days ago. Did you install something, use something, suffer a system crash, rearrange / transport the data in some way at that time?