I was unable to find an installation file for the older versions of the Yahoo Companion Toolbar. But, if you're desperate and/or you consider yourself an advanced user, you can try this:
First, download one of the following files:
Version 5.0.2.6
ftp://4.4.11.227/Program%20Files/Yahoo%21...comp5_0_2_6.dll
Version 5.1.5.0
ftp://200.11.179.36/Archivos%20de%20progr...comp5_1_5_0.dll
Next perform the following steps:
1. If you haven't installed the toolbar yet, install it.
2. You can download an installation file for the lastest version from Zdnet using this link: ftp://ftp.download.com/pub/ppd/10032606-1...tup_cnetPPD.exe
3. Close Internet Explorer.
4. Perform a file search and locate the file ycomp5_3_16_0.dll ( Note: the file may have different numbers following "ycomp" depending on the version you have installed.).
5. The file ycomp5_3_16_0.dll is typically located in one of the following directories: "C:\Windows\Downloaded Program Files" or "C:\Program Files\Yahoo!\Companion\Installs \cpn".
6. Rename ycomp5_3_16_0.dll to ycomp5_3_16_0.bak.
7. Copy one of the older file versions you downloaded above to the same directory where you found ycomp5_3_16_0.dll.
8. Rename the older file to ycomp5_3_16_0.dll.
If all goes well, you should now have the older version of the toolbar when you restart Internet Explorer.
Good luck!