If you saved the file you were working on simply open it and click the Apply Multires button under the Multires tab. Since you don't have any multires levels active in the model this will clear the error msg without causing any actual change to your mesh allowing you to proceed with editing.

The problem on the new file you created appears to be a video issue, google for information related to problems between your video card and Blender to determine if this is an ongoing problem with a solution or just a hiccup in the flow that may or may not repeat itself. I had a couple of oddities show up with Blender 2.48 on my Nvidia 280GTX card that were resolved either with the 2.48a release or the update to my video drivers, unsure which as I did both updates at the same time.