You may have sometimes tried multiple Virus programs to remove all the Viruses from your m/c as a typical program detects only 30% of all threats. Some times even on removing the Virus Program the ensuing installations may detect the previous Software . This may happen because the registry may have some residual keys related to the software which is not removed while uninstalling from Add/Remove Programs option.
Hence to remove these last traces click start/Run , type Regedit and press enter, registry editor will be opened. Search for the program for example in the find option menu type AVG 8 and search for keys . Locate the entry key by Right Clicking and selecting the locate key option and delete the entry. This will now allow you to install the other programs. Also delete other not required entries of uninstalled program to reduce the size and load of the registry.