I went through this on my g/f's stock Evo and we talked about it another thread.
Originally Posted by jnadelman
To fix my g/f's market issues, i powered off the phone, removed the sd card, rebooted, installed an app from market (angry birds seasons), powered down the phone, re-inserted the card and she was fine.
I've read that others have had to format their SD cards completely (after backing up their data to a pc) and I remember someone saying they just erased the android_secure folder - you may want to try to erase the whole folder and see if that cleans up, or do what I did with removing the SD card, or if that all fails backup the card, partition it and format it. (I know I didn't say partition earlier, but if you're backing up the whole card you might as well set it up nice for Android).