Well avast 5/6 are reportedly lighter than avast 4.8.
Your biggest issue is getting the latest version of either avast5 or avast6, download sites like filehippo or majorgeeks or oldversion.com should have copies of the old versions of avast.
You would need the win2k rollup1 update mentioned in my last post and most certainly need C++ 2008:
You need to install MS Visual C++ 2008 Redistributable.
You need 2008 SP1 - this is the one:Microsoft Visual C++ 2008 SP1 Redistributable Package ATL Security Update
As I mentioned the helpers on the forums haven't used the old avast versions for some considerable time, so to give detailed information on how to proceed other than to get the elements needed would be very limited. The problem is I don't know if C++ 2008 is compatible with win2k or if you would need C++ 2005 (which would probably limit you to avast5). This is the issue with retaining an old version, people just can't remember, so at the very least you would need someone that still uses win2k
You need to get rollup 1, C++ 2008 (or C++ 2005) and the version of avast you are going to install saved to your hard disk, uninstall avast 4.8, reboot install the win2k rollup1 update and what ever version of C++ required, reboot and then install the avast version you have chosen.