It sounds like there might be a fault with the card but, to eliminate the possibility of a fault with Picasa, try copying the pictures from your card to your PC without using that program.
If you need instructions:
Right-click on your desktop and select 'New Folder'. Name it as 'Imported Pics' (or similar). Double-click on the folder, to display its (currently non-existent) contents. If necessary, resize the window, so that it only takes up part of the screen.
Then double-click on 'My Computer' and again on the icon for your memory card. (It will probably be 'Drive G', or similar. If there are several possible drives, just try them each in turn, until a window opens displaying the contents of your card. It's likely that you'll only see a folder displayed, if so, double-click on it - and on the next folder if necessary - until you actually get to your photo files). If necessary, resize the new window, so that you now have two windows open, side by side.
In the second window go the the Edit menu, and click on 'Select All'. Now 'drag-and-drop' the files from that window to the empty one.
You've now got the pictures onto your PC. Double-click on one of the files in the 'Imported Pics' window, to see if it will open in your default picture viewing program.
Chris