DirectX Disappeared

I played FFXI last night, shut down my computer, no issues.
Today, I turn it on, try to boot up FFXI, and it tells me DirectX is not installed. What the...? I checked my HD, it really is GONE. Not even a trace.  I redownloaded the software, but still. How can a program disappear without a trace? I am so goddamn confused. I ran my virus scan just to be safe, and I'm clean.

>>6
I've used Windows PC's all my life, never had an issue like this.
And if you want to get into the "zomg windoze sux" debate, well, if you know how to do basic system maitenance, it'll run as well as a Mac. Just because you are incompentent doen't mean the OS is bad. In other words, explain or GTFO.