This problem really sucks.... but there is a solution (This works on a x86 Vista. x64 should be similar).
1. Download the Visual C++ 2008 redistributable package from Microsoft at http://www.microsoft.com/downloads/details.aspx?FamilyID=9b2da534-3e03-4391-8a4d-074b9f2bc1bf&displaylang=en#Requirements
2. Attempt to execute it. It should extract the files and install it.
3. Now try to start Avast. If it doesn't work and you get the side by side error, continue.
4. Create a folder called "ms" (without the quotes) on your root directory of your hard drive ex. c:\ms
5. Copy the file you downloaded into that directory
6. Go to the command line with administrator privileges.
6a. If you don't know how to do it, go to All Programs > Accessories. Find Command Prompt. Hover over it. Right click and click on "Run as administrator".
7. Type the following and press enter: cd \ms
8. Type the following and press enter: vcredist_x86.exe -x
9. It will ask you for a directory to save extracted files. Type in c:\ms\extract and click OK.
10. Files will be extracted. Close the command prompt window.
11. Using file explorer go to c:\ms\extract
12. Double click on vc_red.msi (if you have extensions hidden, execute the vc_red that has a type "Windows Installer Package").
13. Installer package will install.
14. Delete c:\ms as you will no longer need it.