After installing Oracle Developer Suite 10 g for Win 32 (Oracle client gets also installed), I installed a separate Oracle client 10.2.0.1.0. I made the first Oracle home the Default home by setting the Path. Connection via first Oracle home works OK. However, choosing the second Oracle home gives an Access Violation error.
Try adjusting the Environmental Variables PATH setting:
1. Go to your My Computer Icon, right click.
2. Select Properties | Advanced | Environmental Variables.
3. Under the System Variables section, find the PATH.
PATH should begin with the path to the intended client's bin folder, which holds the client's dlls. If this is not already found in the front, edit PATH and add the client's bin path to the front of the string.
EX: Wish to use client CLI10gR2. The client's dlls are located in C:\Oracle\CLI10gR2\Bin;
PATH should be: C:\Oracle\CLI10gR2\Bin;C:\Oracle\CLI9i\Bin;...
Waiting for a fix in a future release of Toad for Oracle.