I've had similar situations before....

Sadly, the only thing that fixed it in my case was a recovery. Do you have a recovery partition or CDs for this computer?

I do have one idea that you can try: Try running a chkdsk. (Start>My computer>Right click C:\>Properties>Tools>Error checking>check both boxes>OK>OK>Reboot).

Also, try swapping out the DVD drives for known good ones.

Its odd that it happened to both of these computers....

Post back with results!

Let me know if you need anything clarified.

--The Commodore