I was cleaning a friend's HP with Vista Home Premium as the OS. This computer hadn't been maintained since it was new 6 years ago. I cleaned out 30 gigs just out of the recycle bin, not counting many junk programs downloaded through the years, empty files, etc. I ran scan disc, a registry cleaner and defragger and just basic housekeeping. Everything was working fine, computer ran much faster. I was running the final defrag, and during the middle of it, pop-ups from yahoo messenger and Nortons came up. I was sure they were both disabled but ???. I then got windowexplorer.exe. errors, my defragg quit, and the cursor keeps spinning like it's working and never stops and now doesn't work. The fault from windowsexplorer is fromStackHash.b7a6. I went into task mgr and tried to close out anything running. I still had the spinning cursor and error messages. I than started it in safe mode but the screen flashes like a strobe light. I than tried the recovery programs, no improvement but I was able to see the resource monitors and both the cpu and memory are at 100%, but the task manager doesn't show anything any processes or services using over 3-4%. I'm pulling out my hair because it's not my machine and I'm out of ideas. Please help.