Here's my horror stories.
Back when I was running Windows 98, I purchased a program called "Windows Cleaner" or something like that, from a computer store bargain bin, I came home installed it, ran it with out knowing anything about it and wiped out half my registry. Ouch, had to reformat.
Again, with Windows 98, I once installed a utilities program, which offered a "disk clean option", Ran the disk clean, and cleaned out some core Win98 files.
A while back when I had first bought XP, I decided to un-install the Microsoft Games (Backgammon, Solitaire, Checkers, etc). I went through the Add/Remove Programs and uninstalled all the MS games as well as Internet Explorer, Outlook Express and Windows Explorer.
Once when XP was relatively new, I installed NAV, and then later on, installed another anti virus program as well (some freeware one can't remember the name). NAV started a scheduled scan while the other one was scanning. They both scanned for about 3 seconds and then the whole system crashed. I got the blue screen of death and could not reboot.
Anyone else buggered up their system?
Interested to here your stories.