user6542: 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)

user12835: The only thing you need keep between installations of bitcoin is the wallet.dat file, which contains the keys required to spend your coins. If you are reinstalling bitcoin you can use bootstrap.dat in versions after 0.7.1 (at the time of writing the current stable version is 0.8.6).
Here's how to use bootstrap.dat to avoid the painfully long block chain download for a new installation: https://bitcointalk.org/index.php?topic=145386.0
Even if you've already downloaded some blocks, bitcoin will check for bootstrap.dat and sync. At the time of writing the file is 13+ gb, but you can delete it once that your client has verified all the blocks.

user6542: 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

6542: <h2>When I try to load my Bitcoin Wallet I get an "Assertion Failed!" window:  here is what the window says:</h2>

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

<p>Expression: pfork !=NULL</p>

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

<p>(Press Retry to debug the application - JIT must be enabled)</p>

<h2>(three buttons:)     Abort     Retry    Ignore</h2>

<p>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.</p>

<p>Many, many thanks for any help you can give.
Sincerely
Douglas (Scotland, UK)</p>
 12835: <p>The only thing you need keep between installations of bitcoin is the wallet.dat file, which contains the keys required to spend your coins. If you are reinstalling bitcoin you can use bootstrap.dat in versions after 0.7.1 (at the time of writing the current stable version is 0.8.6).</p>

<p>Here's how to use bootstrap.dat to avoid the painfully long block chain download for a new installation: <a href="https://bitcointalk.org/index.php?topic=145386.0" rel="nofollow">https://bitcointalk.org/index.php?topic=145386.0</a></p>

<p>Even if you've already downloaded some blocks, bitcoin will check for bootstrap.dat and sync. At the time of writing the file is 13+ gb, but you can delete it once that your client has verified all the blocks.</p>
 6542: <p>Found out a way to fix the problem - really easy too:</p>

<p>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.</p>

<p>Thank you all for your help.
D</p>
