I think what Asyn was asking was which version of C++ you installed. There are both versions on the MS download site.
Here's what you have to do. You have to uninstall Avast!, then install the C++ package, then reinstall Avast!. I had the exact same problem on the Vista machine here and that was how I fixed it. I first installed the C++ files with Avast! still installed and then the Avast! GUI would open but I couldn't get it to update the virus defs correctly. It said I needed to update but then the updater said I had the latest defs. The only thing that fixed it was to uninstall Avast! completely, including using the removal tool, rebooting and then reinstalling Avast!. Ever since then it's been working perfectly. For some reason, the Avast! 5 installer does not always properly install the needed C++ files.
Once you get everything working, go to Windows update and see if there is an update for the C++2008 package, there almost certainly will be one that shows up and it's an important security fix.
You really should do this and get v5 working. It's much better than v4.8 was.