Please try to download full installation file of AVG on another computer, move it to your computer over USB drive and attempt to install it in safe mode.
Installation instructions and link to full installation file:
If it still doesnt help please follow this thread:
Computer was infected with malware "Vista Internet security 2010" and was blocking internet. By going onto another user ID on the computer managed to download AVG free 9.00 but didn't cure problem. Restored system to 2 days previous and got ris of Malware. But it also has got reid of my AVG 8.5 that I had and comes up with installation failed. When tr to download, the files do not get written to the PC. Also tried selecting the free trial on the pay version and get the same problems. Tried running and saving to desktop options. opertaing system is Vista. Any help greatly appreciated