Do you know which version of .Net is having the problem? Anything in Event Viewer about the problem.
If you system is Blue Screening, you might check the BSOD forum to see how to set up the system to create dump files you could attach.
In Programs and Features, if .Net 4 was involved, there is an option to repair it. The earlier .Net installs are, I believe, part of Windows 7 basic install.