I always use the clean/green versions of QQ on my PC, because the original/full versions are huge mess. Once again, Tencent forces us to upgrade to the latest version, QQ2013. Here is the clean version of it:
Unzip the downloaded 7z file with 7z, to anywhere you prefer, such as C:\QQ2013. Then go into the BIN folder and double click on QQ.exe.
If you were using QQ, you may copy the "Users" folder from the old location to this new location. Then your old messages are all still available in this new version.
If you need to copy your QQ from another computer, then you also need to copy the "Tencent" and "Tencent Files" folders from the "Documents" folder to your current "Documents" folder. Furthermore, copy the "Tencent" folder from "C:\Users\UserName\AppData\Roaming" on the old computer to the similar location on the new computer.