After a crash, I just reinstalled my OS: Win XP64, sp2. In addition to reinstalling other software, I also donwloaded the most recent Free Anti Virus (as of May 23, 2015), and attempted to install. Progress bar got all the way across, then hung (forever; tried 3 times). The destination directory appeared full, but the desktop icon was placed, but empty. "Computer Admin | add/remove programs didn't show avast. Cntr-Alt-Del couldn't kill the installation.
The only way out was to reboot. But this caused even worse problem: it trashed my video driver (NVIDIO, gfx660Ti; three times reproducibly!), requiring either reinstallation of video driver in "safe mode" or restoring boot drive from back-up image.
I'm not keen on trying again, until someone has some really convincing ideas on how to circumvent the problem.
Thnaks for any help,
Bob W.