Bitcoin QT always out of sync

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.