I can top that, one of my most time consuming jobs was a customer who said his computer was acting strange. He brought it to me and I turned it on, it was a Windows XP box. The first thing that caught my attention was that there were multiple AV programs installed. There were AV programs I hadn't even heard of at the time installed, giving him a grand total of about ten. Some of them very pricey, corporate versions that no EU would be expected to configure and run. Within about three minutes of being up the box tried and failed to access the internet and send over a thousand emails. The error messages were popping up so fast they were overloading the processing power of the machine. Fortunately XP is about as insecure as anything you'd imagine as long as you're infront of it. After bypassing the infected account and deleting all but one of the AV programs I found two things, that not one of them had ever been run, and that Windows Update, Windows Firewall, and several services had been disabled, and from the last run notes, had been disabled since the EU unboxed his machine. The AV programs couldn't run because the Server service had been disabled. After enabling it, the sole remaining AV found thousands of threats. Ordinarily a box this badly comprimised would have been considered a lost cause but he had no backup of any type of any of his data. Twelve hours of updates and cleaning later, the box was stable. When I asked him why he'd turned off the services he had, he responded that he wanted his machine to run faster and he'd read somewhere that he could do that by disabling unnecessary services he didn't use.
He is a die hard Microsoft Customer, and has now moved on to Windows 8 and it has already been extremely comprimised twice, once resulting in the total loss of everything on his system. The best thing any user can do regardless of your OS of choice is find out what the best practices and standards are, and here's the important part follow them. There are lots of ways to comprimise any system, regardless of what security measures you've taken, there are also lots of ways to keep your system from being comprimised. Congrats to the original poster for doing the right thing and asking the question. It seems that these discussions don't happen enough.