Well, I'll just quote myself from the Creative Cow!
There aren't any effects applied to your still, so you couldn't have inadvertently used the Skew effect. I'm left with concluding that you have Open GL acceleration enabled for rendering, which is almost always a bad thing.
Adobe employees don't use it, so you should turn it off if it's on.
But if you're really new, and you're trying to go it alone, there is help available and you really should take advantage of it:
You might be able to dope out how to insert a table into a Microsoft Word document just from the Help section. But as surely as day follows night, trying to learn AE without good training materials -- plus beginning at the beginning -- is a sure recipe for wasting time, frustration, and overlooking valuable features.
This kind of skewing is a CoDec issue in the final output or a hardware issue in whatever player software you use. The long and short of it is, that a discrepancy between the timebase info and the actual pixel data leads to wrong decoding. There is/ was also a known bug with some Quicktime stuff, where AE would wrongly add an extra pixel at the end of each line, but not account for this pixel in teh file's resolution info, thus leading to the skewing because the pixels accumulated an increasing shift per line. Might even relate to meta data in the file. I can't remeber the exact specifics. in any case, use anotehr output format and experiment with your settings and aslo do check your player software's hardware acceleration options for potential issues.