How to make monitor edges match on Windows 7
Natively, you can't get rid of the unaligned edges unless you set the displays to the same resolution, and align the displays (on the side they share) in the display properties.
The program DisplayFusion has an option to prevent bumping against edges when moving between monitors at different resolutions, as well as ones that are not aligned exactly.
I've been happily using DF for years to deal with multiple monitors, and it's handy as heck. :)