I'm going to assume that an ActiveX checkbox is what you want...
It sounds like the checkbox's wrapping is set to In line with text. There
are [at least] a couple of options:
1. Right-click the checkbox and choose Format Control. In the Layout tab,
set the Wrapping style to Tight or Square. You should now be able to drag it
so that it is aligned with the text.
2. Put the In line with text checkbox and the text to which it pertains both
into a table. Set the vertical formatting for that row to Centered, and make
sure there is no After spacing in the paragraph formatting for those cells.
When I'm setting up a form, I usually go with option #2. In the long run,
things are a lot more stable and easier to maintain and position when placed
into a table.