"Assertion Failed!" when I try to open Bitcoin Wallet When I try to load my Bitcoin Wallet I get an "Assertion Failed!" window:  here is what the window says:

Program: C:\Program Files\Bitcoin\bitcoin-qt.exe
File: src/main.cpp
Line: 1761

Expression: pfork !=NULL

For information on how your program can cause an assertion failure, see the visual C++ documentation on asserts

(Press Retry to debug the application - JIT must be enabled)

(three buttons:)     Abort     Retry    Ignore

I am a novice and don't know where to start.  I do have a backup on a flash-drive if that is of any help.

Many, many thanks for any help you can give.
Sincerely
Douglas (Scotland, UK)
Found out a way to fix the problem - really easy too:

1) Copy bitcoin data files to external hard drive
2) Uninstall Bitcoin application
3) Copy bitcoin data files back to their original location
4) Download and reinstall Bitcoin application.  It finds the old files and everything works perfectly.

Thank you all for your help.
D