Hey riptele,
Are you getting any error messages or does ArcMap just crash when you try to check out? Are you working with Shapefiles or Feature Classes within your mxd? Have you tried adding the data you are trying to check out to a new, blank mxd and checking it out?
No error messages and no ArcMap crash. We are working with feature classes and we have tried a blank mxd and still no records checked out. The problem started when we moved to 64bit windows 7 and arcgis 10.1.