2013 08 29; 16h42 GMT
I had a similar problem - Assertion failed! windo when tried to open Bitcoin Wallet.  Your solution worked Willie - and I am very, very grateful to you for the advice.  What I did exactly:  1) I had a backup copy of all the data (folders and files) under C:\Documents and Settings\My User Name\Application data\Bitcoin  (Windows XP).   2) Uninstalled Bitcoin  3) copied all the backup data from my external hard disk to the Bitcoin folder indicated above;   4) Downloaded Bitcoin and ran it;   5) All fixed!
Thanks once again - people like you are invaluable - I had just over £10,000 that I thought I had lost - have not slept for two days and nights.